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/propdlg.h

@@ -49,7 +49,7 @@ enum wxPropertySheetDialogFlags
 
     /**
         Shrinks the dialog window to fit the currently selected page
-        (common behaviour for property sheets on OS X).
+        (common behaviour for property sheets on macOS).
     */
     wxPROPSHEET_SHRINKTOFIT = 0x0100,
 };