Update documentation index sidebar for 3.6.0 release.

Doc/tools/templates/indexsidebar.html

@@ -4,7 +4,7 @@
 <ul>
   <li><a href="https://docs.python.org/2.7/">{% trans %}Python 2.7 (stable){% endtrans %}</a></li>
   <li><a href="https://docs.python.org/3.5/">{% trans %}Python 3.5 (stable){% endtrans %}</a></li>
-  <li><a href="https://docs.python.org/3.6/">{% trans %}Python 3.6 (pre-release){% endtrans %}</a></li>
+  <li><a href="https://docs.python.org/3.6/">{% trans %}Python 3.6 (stable){% endtrans %}</a></li>
   <li><a href="https://www.python.org/doc/versions/">{% trans %}Old versions{% endtrans %}</a></li>
 </ul>