Add missing C# cast in first 3D game (#5203)

getting_started/first_3d_game/08.score_and_replay.rst

@@ -429,7 +429,7 @@ Here is the complete ``Main.gd`` script for reference.
 
         public void OnMobTimerTimeout()
         {
-            Mob mob = MobScene.Instance();
+            Mob mob = (Mob)MobScene.Instance();
 
             var mobSpawnLocation = GetNode<PathFollow>("SpawnPath/SpawnLocation");
             mobSpawnLocation.UnitOffset = GD.Randf();