Display the last top level element in the Table of Contents as expandable.

This isn't a great fix, but I'm confused what's going on. It's something
in JQuery.TreeView. We write out the HTML correctly, but the lastExpandable
style causes the list item to not be expandable. This didn't bite us in
6.002x because the last item was an appendix that isn't expandable anyway.
It does affect 6.00x, where the last item is Chapter 5 at the moment.

lms/templates/staticbook.html

@@ -97,6 +97,10 @@ $("#open_close_accordion a").click(function(){
           % for entry in table_of_contents:
             ${print_entry(entry)}
           % endfor
+        
+        ## Don't delete this empty list item. Without it, Jquery.TreeView won't
+        ## render the last list item as expandable.
+        <li></li> 
       </ul>
     </section>