clean up EndEpisode demo code (#4563)

docs/Learning-Environment-Create-New.md

     }
 
     // Fell off platform
-    if (this.transform.localPosition.y < 0)
+    else if (this.transform.localPosition.y < 0)
     {
         EndEpisode();
     }

docs/Learning-Environment-Design-Agents.md

     AddReward(1.0f);
     EndEpisode();
 }
-if (hitObjects.Where(col => col.gameObject.tag == "pit").ToArray().Length == 1)
+else if (hitObjects.Where(col => col.gameObject.tag == "pit").ToArray().Length == 1)
 {
     AddReward(-1f);
     EndEpisode();