Fix typo in README
This is the correct name since 2016.
@@ -26,7 +26,7 @@ If you require the latest version in development, it should be possible to
compile RGBDS with MinGW or Cygwin by following the instructions to build it
on UNIX systems.
-### 1.2 Mac OS
+### 1.2 macOS
You can build RGBDS by following the instructions below. However, if you would
prefer not to build RGBDS yourself, you may also install it using
