Fix type of 'what' parameter to _Notifications In C# usage example (#11008)

* Fix type of 'what' parameter to _Notifications In C# usage example

tutorials/best_practices/godot_notifications.rst

@@ -360,9 +360,9 @@ nodes that one might create at runtime.
             return _parentCache.HasUserSignal("InteractedWith");
         }
 
-        public void _Notification(int what)
+        public override void _Notification(int what)
         {
-            switch (what)
+            switch ((long)what)
             {
                 case NotificationParented:
                     _parentCache = GetParent();