docs: Don't let Doxygen expand HOME environment variable

Otherwise, the API documentation will encode the home directory of the
user or autobuilder that built SDL, instead of telling the user to use
the literal string $(HOME) as intended.

See also <https://github.com/doxygen/doxygen/issues/7073>.

Signed-off-by: Simon McVittie <smcv@debian.org>

docs/README-macos.md

@@ -239,8 +239,8 @@ Some of you won't want to use the Stationary so I'll give some tips:
 * Remove "main.m" from your project
 * Remove "MainMenu.xib" from your project
 * Remove "AppDelegates.*" from your project
-* Add "$(HOME)/Library/Frameworks/SDL.framework/Headers" to include path
-* Add "$(HOME)/Library/Frameworks" to the frameworks search path
+* Add "\$(HOME)/Library/Frameworks/SDL.framework/Headers" to include path
+* Add "\$(HOME)/Library/Frameworks" to the frameworks search path
 * Add "-framework SDL -framework Foundation -framework AppKit" to "OTHER_LDFLAGS"
 * Add your files
 * Clean and build