Fix name of 'macOS' and add asteriks to brands, NFC.

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@323180 91177308-0d34-0410-b5e6-96231b3b80d8

Author: hahnjo

README.rst

@@ -194,7 +194,7 @@ Optional Features
 **LIBOMP_OMPT_SUPPORT** = ``ON|OFF``
   Include support for the OpenMP Tools Interface (OMPT). 
   This option is supported and ``ON`` by default for x86, x86_64, AArch64, and 
-  PPC64 on Linux, Windows, and mac OS.
+  PPC64 on Linux*, Windows*, and macOS*.
   This option is ``OFF`` if this feature is not supported for the platform.
 
 **LIBOMP_OMPT_OPTIONAL** = ``ON|OFF``