Update PhysicsServer2D naming for C# in ray-casting.rst

Current documentation refers to "Physics2DServer" exclusively in C# code snippet, when the actual name should be PhysicsServer2D.

(cherry picked from commit 5114103e23302f36b0db7a2aec2fe96a51db0d14)

tutorials/physics/ray-casting.rst

@@ -63,7 +63,7 @@ Use the following code in 2D:
     public override void _PhysicsProcess(double delta)
     {
         var spaceRid = GetWorld2D().Space;
-        var spaceState = Physics2DServer.SpaceGetDirectState(spaceRid);
+        var spaceState = PhysicsServer2D.SpaceGetDirectState(spaceRid);
     }
 
 Or more directly: