don't inline aria-description example to appease w3c validator2

Author: davidcarlisle

src/accessibility.html

@@ -441,7 +441,7 @@ <h5  id="acc_descriptions">Accessible Descriptions</h5>
   with each non-trivial component described.
 </p>
 
-<div class="example mathml">
+<div class="example mathml-fragment">
 <pre>
 &lt;mrow aria-description="circumference of a circle"&gt;
   &lt;mn&gt;2&lt;/mn&gt;