xrandr-capplet: Fix error -Wincompatible-pointer-types

based in mate-control-center commit:
mate-desktop/mate-control-center@f164a47

Fixes the error:
xrandr-capplet.c: In function 'run_application':
/usr/include/glib-2.0/gobject/gtype.h:2660:6: error: passing argument 1 of 'ctk_show_uri_on_window' from incompatible pointer type [-Wincompatible-pointer-types]

capplets/display/xrandr-capplet.c

@@ -2499,7 +2499,7 @@ run_application (App *app)
 	break;
 
     case CTK_RESPONSE_HELP:
-        ctk_show_uri_on_window (CTK_DIALOG (app->dialog),
+        ctk_show_uri_on_window (CTK_WINDOW (app->dialog),
                                 "help:cafe-user-guide/goscustdesk-70",
                                 ctk_get_current_event_time (),
                                 &error);