fix link to contributing in website drop-down

Author: amueller

doc/themes/scikit-learn/layout.html

@@ -76,7 +76,7 @@
             <li><a href="{{ pathto('user_guide') }}">User guide</a></li>
             <li><a href="{{ pathto('modules/classes') }}">API</a></li>
             <li><a href="{{ pathto('faq') }}">FAQ</a></li>
-            <li><a href="{{ pathto('developers/index') }}">Contributing</a></li>
+            <li><a href="{{ pathto('developers/contributing') }}">Contributing</a></li>
             <li class="divider"></li>
                 <li><a href="http://scikit-learn.org/stable/documentation.html">Scikit-learn 0.16.1 (stable)</a></li>
                 <li><a href="http://scikit-learn.org/0.15/documentation.html">Scikit-learn 0.15</a></li>