Fix color on example btn on navbar (#24052)

docs/4.0/components/navbar.md

@@ -323,7 +323,7 @@ Theming the navbar has never been easier thanks to the combination of theming cl
       </ul>
       <form class="form-inline">
         <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
-        <button class="btn btn-outline-secondary my-2 my-sm-0" type="submit">Search</button>
+        <button class="btn btn-outline-light my-2 my-sm-0" type="submit">Search</button>
       </form>
     </div>
   </nav>