Docs: Fix pluralization in CMake docs

Fixed "most platform" to read "most platforms", and changed "are not
using the same" to "use different" as a drive-by.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303487 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed