[doc] about page updates

searx/templates/about.html

 
 <h3>New engines?</h3>
 <ul>
-    <li>Edit your engines.cfg, see <a href="https://raw.github.com/asciimoo/searx/master/engines.cfg_sample">sample config</a></li>
+    <li>Edit your <a href="https://raw.github.com/asciimoo/searx/master/settings.yml">settings.yml</a></li>
     <li>Create your custom engine module, check the <a href="https://github.com/asciimoo/searx/blob/master/examples/basic_engine.py">example engine</a></li>
 </ul>
 <p>Don't forget to restart searx after config edit!</p>