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/aboutdlg.h

@@ -230,7 +230,7 @@ public:
         "Version " to @a version).
 
         The generic about dialog and native GTK+ dialog use @a version only,
-        as a suffix to the program name. The native MSW and OS X about dialogs
+        as a suffix to the program name. The native MSW and macOS about dialogs
         use the long version.
     */
     void SetVersion(const wxString& version, const wxString& longVersion = wxString());