footer: Apply aria-hidden to whole button. (#43)

Apply aria-hidden to the whole span which contains the back to top span.

Signed-off-by: Stephan Kulla <git.mail@kulla.me>

layouts/partials/footer_container.html

@@ -7,10 +7,10 @@
       Powered by the <a href="https://github.com/gcushen/hugo-academic" target="_blank">Academic
       theme</a> for <a href="http://gohugo.io" target="_blank">Hugo</a>.
 
-      <span class="pull-right">
+      <span class="pull-right" aria-hidden="true">
         <a href="#" id="back_to_top">
           <span class="button_icon">
-            <i class="fa fa-chevron-up fa-2x" aria-hidden="true"></i>
+            <i class="fa fa-chevron-up fa-2x"></i>
           </span>
         </a>
       </span>