README-migration: Added backticks to avoid markdown highlighting.

docs/README-migration.md

@@ -432,7 +432,7 @@ The following functions have been renamed:
 
 ## SDL_platform.h
 
-The preprocessor symbol __MACOSX__ has been renamed __MACOS__, and __IPHONEOS__ has been renamed __IOS__
+The preprocessor symbol `__MACOSX__` has been renamed `__MACOS__`, and `__IPHONEOS__` has been renamed `__IOS__`
 
 ## SDL_rect.h