documentation: change ascii ' quotes to either ’ or _

Non-reference AsciiDoc implementations such as AsciiDoctor (used by
GitHub) or even the simplistic parser in loccount choke up on these
characters for different reasons.  AsciiDoctor doesn’t seem to even
parse them, displaying them as-is and not with emphasized text.
loccount would get hung up on possessives, expecting a terminating
character for emphasis that never comes.

Annoying, but easy to work around.  the curly ’ character can be used
for possessives and _underscores_ can be used for emphasis.

README.adoc

@@ -48,7 +48,7 @@ https://zdoom.org/[GZDoom].  This engine offers good support for
 single-player, multi-player, and the majority of mods created for both
 _Doom_ and _Freedoom_.
 
-On Windows, you should place _Freedoom_'s data files (those ending
+On Windows, you should place _Freedoom_’s data files (those ending
 with +.wad+) alongside the engine (eg, +odamex.exe+).  On Unix-like
 systems, these data files should go in either +/usr/share/games/doom+
 or your home directory.  If _Freedoom_ comes packaged as part of your
@@ -72,10 +72,10 @@ of the following options should solve it:
     line, or adding it to an application shortcut.
   * Use the +DOOMWADPATH+ environment variable.  Many engines support
     this variable to add directories and/or files to their search
-    path.  The exact syntax matches your operating system's normal
+    path.  The exact syntax matches your operating system’s normal
     +PATH+ environment variable.
   * Rename the game files.  This may be a bit crude, but you can
-    rename the files to match those of _Doom_'s.  This is often the
+    rename the files to match those of _Doom_’s.  This is often the
     easiest quick-fix, although it is normally desirable to use one of
     the above methods if possible.
 
@@ -147,7 +147,7 @@ The general rules go as follows:
 Levels for _Phase 1_ and _Phase 2_ must be compatible with any limit
 removing engine.  This means that you may exceed the limits of the
 original _Doom_, but do not depend on any additional mapping features.
-Levels should be in _Doom_'s original format, not in “Hexen”-format.
+Levels should be in _Doom_’s original format, not in “Hexen”-format.
 
 It is a goal that future versions of _Freedoom_ will be entirely
 vanilla-compatible, not even allowing expanded limits.  Keeping this
@@ -169,7 +169,7 @@ It is sensible to also heed the following guidelines:
   * Try to make levels appropriately difficult for their position
     within the progression of the game.  Also bear in mind that not
     all players may be as skilled a player as you.
-  * Do not use tricks that exploit _Doom_'s software renderer; some
+  * Do not use tricks that exploit _Doom_’s software renderer; some
     engines, especially those that use hardware accelerated rendering,
     may not render it properly.  Examples of tricks to avoid include
     those used to simulate 3D bridges and “deep water” effects.
@@ -193,7 +193,7 @@ It is sensible to also heed the following guidelines:
 Graphics should generally have the same color and size as the original
 _Doom_ graphics, as to remain compatible with mods.  Otherwise, levels
 may end up looking like a nightmare in design.  They may be
-thematically different as long as it doesn't clash.
+thematically different as long as it doesn’t clash.
 
 _Doom_ uses a fictional corporation abbreviated as “UAC:” this is
 trademarked by id Software and cannot be used in _Freedoom_.  Instead,
@@ -212,7 +212,7 @@ patches, but keep in mind:
     for Vim (perhaps other editors, too) to automatically re-format
     text.
   * Text is kept at 72 characters wide.  In Vim, you can set the
-    editor to automatically insert line breaks as you're typing by
+    editor to automatically insert line breaks as you’re typing by
     performing `set textwidth=72`.  Special exceptions to the width
     rule might be allowed when necessary (for example, inserting long
     URLs).
@@ -252,7 +252,7 @@ You can also commit on a clone of the _Freedoom_ repository, although
 this is a technical task and it is okay to let other _Freedoom_
 maintainers to do it instead: that is our normal mode of operation.
 However, pull requests are much appreciated and you may submit them in
-any manner you wish, with GitHub's direct pull requests being the
+any manner you wish, with GitHub’s direct pull requests being the
 simplest, but by far not the only means.
 
 Freedoom uses the commit message style commonly seen in distributed