Use correct class for header in jumbotron dropdown nav

jumbotron/index.html

@@ -37,7 +37,7 @@
                 <li><a href="#">Another action</a></li>
                 <li><a href="#">Something else here</a></li>
                 <li class="divider"></li>
-                <li class="nav-header">Nav header</li>
+                <li class="dropdown-header">Nav header</li>
                 <li><a href="#">Separated link</a></li>
                 <li><a href="#">One more separated link</a></li>
               </ul>