fix MissingReferenceException in GridWorld (#3346)

Project/Assets/ML-Agents/Examples/GridWorld/Scripts/GridAgent.cs

                 SetReward(1f);
                 Done();
             }
-            if (hit.Where(col => col.gameObject.CompareTag("pit")).ToArray().Length == 1)
+            else if (hit.Where(col => col.gameObject.CompareTag("pit")).ToArray().Length == 1)
             {
                 SetReward(-1f);
                 Done();