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/toolbook.h

@@ -28,7 +28,7 @@ wxEventType wxEVT_TOOLBOOK_PAGE_CHANGING;
 
     @beginStyleTable
     @style{wxTBK_BUTTONBAR}
-        Use wxButtonToolBar-based implementation under OS X (ignored under
+        Use wxButtonToolBar-based implementation under macOS (ignored under
         other platforms).
     @style{wxTBK_HORZ_LAYOUT}
         Shows the text and the icons alongside, not vertically stacked (only