Use correct dropdown header class in sticky-footer

"nav-header" does not exist in Bootstrap. The class used for headers
in dropdowns is "dropdown-header".

sticky-footer-navbar/index.html

@@ -41,7 +41,7 @@
                   <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>