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/html/webkit.h

@@ -20,7 +20,7 @@
     RunScript return a value, which is a very critical feature in many web
     embedding scenarios.
 
-    This class is only available on OS X.
+    This class is only available on macOS.
 **/
 
 class wxWebKitCtrl : public wxControl