docs: Don't use clutter_stage_get_default()

clutter_stage_get_new() is the recommended way to
get a stage instance, so use that instead.

doc/cookbook/examples/actors-composite-main.c

@@ -34,7 +34,7 @@ main (int   argc,
 
   clutter_init (&argc, &argv);
 
-  stage = clutter_stage_get_default ();
+  stage = clutter_stage_new ();
   clutter_actor_set_size (stage, 400, 400);
   clutter_stage_set_color (CLUTTER_STAGE (stage), &stage_color);
   g_signal_connect (stage, "destroy", G_CALLBACK (clutter_main_quit), NULL);