Add pi_tutor index

Author: h-east

_layouts/vimdoc.html

@@ -277,6 +277,7 @@
 <li><a href="pi_paren.html">pi_paren</a></li>
 <li><a href="pi_spec.html">pi_spec</a></li>
 <li><a href="pi_tar.html">pi_tar</a></li>
+<li><a href="pi_tutor.html">pi_tutor</a></li>
 <li><a href="pi_vimball.html">pi_vimball</a></li>
 <li><a href="pi_zip.html">pi_zip</a></li>
 </ul></dd>