Update C# code example

Use the `Root` property instead of the deprecated `GetRoot` method.
`PropagateNotification` accepts an `int` and not a `long`, and so `NotificationWmCloseRequest` needs to be cast to `int`.

tutorials/inputs/handling_quit_requests.rst

@@ -82,7 +82,7 @@ program termination, you should send the notification yourself:
 
  .. code-tab:: csharp
 
-    GetTree().GetRoot().PropagateNotification(NotificationWmCloseRequest)
+    GetTree().Root.PropagateNotification((int)NotificationWmCloseRequest)
 
 Sending this notification will inform all nodes about the program termination,
 but will not terminate the program itself *unlike in 3.X*. In order to achieve