Fix typo

Author: h-east

_layouts/vimdoc.html

@@ -212,7 +212,7 @@
 <li><a href="hebrew.html">hebrew</a></li>
 <li><a href="russian.html">russian</a></li>
 <li><a href="hangulin.html">hangulin</a></li>
-<li><a href="vietnamese.txt">vietnamese</a></li>
+<li><a href="vietnamese.html">vietnamese</a></li>
 </ul></dd>
 
 <dt>GUI</dt>