docs(examples): ensure content sits in landmarks (#31579)

site/content/docs/5.0/examples/dashboard/index.html

@@ -11,7 +11,7 @@ extra_js:
   - src: "dashboard.js"
 ---
 
-<nav class="navbar navbar-dark sticky-top bg-dark flex-md-nowrap p-0 shadow">
+<header class="navbar navbar-dark sticky-top bg-dark flex-md-nowrap p-0 shadow">
   <a class="navbar-brand col-md-3 col-lg-2 mr-0 px-3" href="#">Company name</a>
   <button class="navbar-toggler position-absolute d-md-none collapsed" type="button" data-toggle="collapse" data-target="#sidebarMenu" aria-controls="sidebarMenu" aria-expanded="false" aria-label="Toggle navigation">
     <span class="navbar-toggler-icon"></span>
@@ -22,7 +22,7 @@ extra_js:
       <a class="nav-link" href="#">Sign out</a>
     </li>
   </ul>
-</nav>
+</header>
 
 <div class="container-fluid">
   <div class="row">