Add C# `using` dependencies to Your first game (#4553)

getting_started/step_by_step/your_first_game.rst

@@ -181,6 +181,9 @@ Start by declaring the member variables this object will need:
 
  .. code-tab:: csharp
 
+    using Godot;
+    using System;
+    
     public class Player : Area2D
     {
         [Export]