fixes invalid aria-role dialogalert which should be alertdialog

lms/templates/modal/accessible_confirm.html

@@ -18,7 +18,7 @@
         </span>
       </h2>
     </header>
-    <div role="dialogalert" class="status message" tabindex="-1">
+    <div role="alertdialog" class="status message" tabindex="-1">
         <p class="message-title"></p>
     </div>
     <hr aria-hidden="true" />