Update C# code example to fix an error

Update doc/classes/AnimationNodeStateMachinePlayback.xml

Co-authored-by: Raul Santos <[email protected]>

doc/classes/AnimationNodeStateMachinePlayback.xml

@@ -12,7 +12,7 @@
 		state_machine.travel("some_state")
 		[/gdscript]
 		[csharp]
-		var stateMachine = GetNode<AnimationTree>("AnimationTree").Get("parameters/playback") as AnimationNodeStateMachinePlayback;
+		var stateMachine = GetNode<AnimationTree>("AnimationTree").Get("parameters/playback").As<AnimationNodeStateMachinePlayback>();
 		stateMachine.Travel("some_state");
 		[/csharp]
 		[/codeblocks]