correcting typo in peer assessment waiting message copy

apps/openassessment/templates/openassessmentblock/peer/oa_peer_waiting.html

@@ -25,7 +25,7 @@
             <h3 class="message__title">Waiting for more peer responses</h3>
 
             <div class="message__content">
-              <p>There are currently no responses for you to assess. This should change momentarily. Check back shortly to continue completing this problem. One this and all other steps are completed, you will receive a grade.</p>
+              <p>There are currently no responses for you to assess. This should change momentarily. Check back shortly to continue completing this problem. Once this and all other steps are completed, you will receive a grade.</p>
             </div>
         </div>
     </div>