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

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

@@ -7,7 +7,7 @@ extra_js:
     async: true
 ---
 
-<div class="container py-5">
+<main class="container py-5">
   <h1>Bootstrap and Masonry</h1>
   <p class="lead">Integrate <a href="https://masonry.desandro.com/">Masonry</a> with the Bootstrap grid system and cards component.</p>
 
@@ -102,4 +102,4 @@ extra_js:
     </div>
   </div>
 
-</div>
+</main>