Fix typo in a class name on the footer.

_includes/footer.html

@@ -40,7 +40,7 @@
     </footer>
 
     <!-- Scroll to Top Button (Only visible on small and extra-small screen sizes) -->
-    <div class="scroll-top page-scroll visible-xs visble-sm">
+    <div class="scroll-top page-scroll visible-xs visible-sm">
         <a class="btn btn-primary" href="#page-top">
             <i class="fa fa-chevron-up"></i>
         </a>