OS X -> macOS (#1934)

Fix all documentation references of "OS X" to be "macOS", Apple's new brand.

Code is left unchanged because we're matching .NET Core.

build.sh

@@ -4,5 +4,5 @@ if hash powershell 2>/dev/null; then
     echo 'Continuing with `powershell -noprofile -c Start-PSBuild`'
     powershell -noprofile -c "Import-Module ./build.psm1; Start-PSBuild"
 else
-    echo 'No `powershell`, see docs/building/linux.md or osx.md to build PowerShell!'
+    echo 'No `powershell`, see docs/building/linux.md or macos.md to build PowerShell!'
 fi