AddReward before EpisodeEnd (#4064)

Changed order of AddReward() and EpisodeEnd() in the Rewards example.

docs/Learning-Environment-Design-Agents.md

     Mathf.Abs(gameObject.transform.position.x - area.transform.position.x) > 8f ||
     Mathf.Abs(gameObject.transform.position.z + 5 - area.transform.position.z) > 8)
 {
-    EndEpisode();
+    EndEpisode();
 }
 ```