Fix link to old documentations

This was mistakenly broken in an earlier patch assuming old docs still
had index.html, which they don't.

templates/docs/archive.html

@@ -18,7 +18,7 @@ If you are still using any of these releases, you are strongly advised to upgrad
 <tr{%if forloop.last%} class="lastrow"{%endif%}>
     <td class="colMid">
           <ul>
-            <li><a href="/docs/{{v.tree}}/static/index.html">{{v.treestring}}</a></li>
+            <li><a href="/docs/{{v.tree}}/static/{{v.indexname}}">{{v.treestring}}</a></li>
           </ul>
       </td>
       <td class="colLast">