add proper markup to list group in panels section

docs/docs.html

@@ -4627,8 +4627,18 @@ For example, <code>&lt;section&gt;</code> should be wrapped as inline.
     </div>
 {% highlight html linenos %}
 <div class="panel">
+  <!-- Default panel contents -->
   <div class="panel-heading">Panel heading</div>
-  Panel content
+  <p>...</p>
+
+  <!-- List group -->
+  <ul class="list-group list-group-flush">
+    <li class="list-group-item">Cras justo odio</li>
+    <li class="list-group-item">Dapibus ac facilisis in</li>
+    <li class="list-group-item">Morbi leo risus</li>
+    <li class="list-group-item">Porta ac consectetur ac</li>
+    <li class="list-group-item">Vestibulum at eros</li>
+  </ul>
 </div>
 {% endhighlight %}