Make the text on the peer grading closed page clearer

lms/templates/peer_grading/peer_grading_closed.html

 <section class="container peer-grading-container">
 <h2>Peer Grading</h2>
+    <p>The due date has passed, and
     % if use_for_single_location:
-        <p>Peer grading for this problem is closed at this time.</p>
+        peer grading for this problem is closed at this time.
     %else:
-        <p>Peer grading is closed at this time.</p>
+        peer grading is closed at this time.
     %endif
+    </p>
 </section>