Fix C# code for changing scene (#4523)

tutorials/scripting/change_scenes_manually.rst

@@ -21,11 +21,11 @@ scenes which one instances and adds to the tree at runtime:
 
  .. code-tab:: csharp
 
-    public PackedScene simultaneousScene;
+    public Node simultaneousScene;
 
     public MyClass()
     {
-        simultaneousScene = (PackedScene)ResourceLoader.Load("res://levels/level2.tscn").instance();
+        simultaneousScene = ResourceLoader.Load<PackedScene>("res://levels/level2.tscn").Instance();
     }
 
     public void _AddASceneManually()