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/bitmap.h

@@ -665,7 +665,7 @@ public:
         @endcode
         in your application startup code.
 
-        However under OS X this function uses native image loading and so
+        However under macOS this function uses native image loading and so
         doesn't require wxWidgets PNG support.
 
         @since 2.9.5