README: replace "MacOSX" with "macOS"

In October 2018, the last version of the operating system named "Mac OS X" ended its life cycle.

It's time to change it to "macOS".

Signed-off-by: Gentry Deng <me@gend.moe>

README.md

@@ -31,7 +31,7 @@ An advanced user may require additional or specific package. (Toolchain, SDK, ..
 
 ## Development
 
-To build your own firmware you need a GNU/Linux, BSD or MacOSX system (case
+To build your own firmware you need a GNU/Linux, BSD or macOS system (case
 sensitive filesystem required). Cygwin is unsupported because of the lack of a
 case sensitive file system.