Map 'role="complementary"' to proper aside element

http://blog.paciellogroup.com/2013/02/using-wai-aria-landmarks-2013/#tablex

http://www.w3.org/TR/wai-aria/roles#complementary

_layouts/default.html

@@ -26,7 +26,7 @@
     <div class="container bs-docs-container">
       <div class="row">
         <div class="col-lg-3">
-          <div class="bs-sidebar" role="complementary">
+          <aside class="bs-sidebar" role="complementary">
             <ul class="nav bs-sidenav">
               {% if page.slug == "getting-started" %}
                 {% include nav-getting-started.html %}
@@ -40,7 +40,7 @@
                 {% include nav-customize.html %}
               {% endif %}
             </ul>
-          </div>
+          </aside>
         </div>
         <main class="col-lg-9" role="main">
           {{ content }}