Updated method name in GDScript example

_on_visible_on_screen_notifier_3d_screen_exited is the default name for the method created in GDScript, and is how the function is referred to elsewhere in the instructions. C# might need changing as well.

getting_started/first_3d_game/04.mob_scene.rst

@@ -237,7 +237,7 @@ method. This function destroy the instance it's called on.
 .. tabs::
  .. code-tab:: gdscript GDScript
 
-   func _on_VisibilityNotifier_screen_exited():
+   func _on_visible_on_screen_notifier_3d_screen_exited():
        queue_free()
 
  .. code-tab:: csharp