fix: aria label for "back to top"

Gives 100% Accessibility score on Lighthouse for test site.

wowchemy/layouts/partials/site_footer.html

@@ -27,8 +27,8 @@
     {{ end }}
 
     {{ if not (in (slice "book" "docs" "updates") .Type) }}
-    <span class="float-right" aria-hidden="true">
-      <a href="#" class="back-to-top">
+    <span class="float-right">
+      <a href="#" class="back-to-top" aria-label="Back to top">
         <span class="button_icon">
           <i class="fas fa-chevron-up fa-2x"></i>
         </span>