Update C# examples for Using Viewports documentation

tutorials/rendering/viewports.rst

@@ -181,7 +181,7 @@ it using (for example):
  .. code-tab:: csharp
 
  // Wait until the frame has finished before getting the texture.
- RenderingServer.FramePostDraw += HandlePostDraw;
+ await RenderingServer.Singleton.ToSignal(RenderingServer.SignalName.FramePostDraw);
  // You can get the image after this.
 
 Viewport Container