Fix typo in peer grading messages.

lms/templates/peer_grading/peer_grading.html

@@ -2,7 +2,7 @@
 <%
 nothing_to_grade_message = _(
 """
-{p_tag}You currently do not having any peer grading to do.  In order to have peer grading to do:
+{p_tag}You currently do not have any peer grading to do.  In order to have peer grading to do:
 {ul_tag}
 {li_tag}You need to have submitted a response to a peer grading problem.{end_li_tag}
 {li_tag}The instructor needs to score the essays that are used to help you better understand the grading