Change documentation references from OS X to macOS (#1927)

Since OS X 10.12 it has been named macOS so it makes sense
to reference it in documentation as such, even when it
sometimes refers to older versions which were called (Mac) OS X.

interface/wx/string.h

@@ -681,7 +681,7 @@ public:
 
     /**
         Converts the strings contents to the wide character representation
-        and returns it as a temporary wxWCharBuffer object (Unix and OS X)
+        and returns it as a temporary wxWCharBuffer object (Unix and macOS)
         or returns a pointer to the internal string contents in wide character
         mode (Windows).