Changed last remaining use of "./" to "index.html" when referring to the

index file for the top-level directory.  This makes it easier to use an
unpacked version of the documentation via file: URLs.
This closes SF bug #541257.

Doc/html/about.html

@@ -30,7 +30,7 @@
       </table>
       <b class="navlabel">Up:</b>
       <span class="sectref">
-        <a href="./" title="Python Documentation Index">
+        <a href="index.html" title="Python Documentation Index">
           Python Documentation Index</A></span>
       <br>
     </div>