change release notes to releases

quick fix for #50 by shortening one title. 

could also maybe shorten Reference to API and not a shorten but Develop should be Contribute, but those two changes will probably be a whole discussion.

Author: story645

mpl_sphinx_theme/mpl_nav_bar.html

@@ -18,6 +18,6 @@
     <a class="reference internal nav-link" href="{{ mpl_path('devel/index') }}">Develop</a>
   </li>
   <li class="nav-item">
-    <a class="reference internal nav-link" href="{{ mpl_path('users/release_notes') }}">Release notes</a>
+    <a class="reference internal nav-link" href="{{ mpl_path('users/release_notes') }}">Releases</a>
   </li>
 </ul>