Switch to .btn-secondary for the search button so that it's accessible (#27255)

site/docs/4.1/examples/starter-template/index.html

@@ -46,7 +46,7 @@
         </ul>
         <form class="form-inline my-2 my-lg-0">
           <input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search">
-          <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
+          <button class="btn btn-secondary my-2 my-sm-0" type="submit">Search</button>
         </form>
       </div>
     </nav>