Replace ASCII external link indicator with UTF-8 character in menu

Author: afh

source/layouts/layout.erb

@@ -19,7 +19,7 @@
             <li><a href="docs.html">Docs</a>
             <li><a href="download.html">Download</a>
             <li><a href="contribute.html">Contribute</a>
-            <li><a href="http://wiki.ledger-cli.org/">Wiki -></a>
+            <li><a href="http://wiki.ledger-cli.org/">Wiki<sup>◹</sup></a>
           </ul>
         </div>
       </div>