Fix the cstyle link.

Author: janpgit

docs/index.html

@@ -94,8 +94,10 @@ <h2><a name="materials">Materials</a></h2>
   href="https://github.com/devnull-cz/unix-linux-prog-in-c-src/releases">releases</a>.
   <li><a href="http://mff.devnull.cz/vim-tips.html">Some ViM tips</a> for
   editting and navigating C source code files.  <li>Information on the
-  recommended <a href="http://mff.devnull.cz/cstyle">C style</a> for
-  writing code so that <b>fellow programmers can read it as well.</b>
+  recommended <a
+  href="https://devnull-cz.github.io/unix-linux-prog-in-c/cstyle.html">C
+  style</a> for writing code so that <b>fellow programmers can read it as
+  well.</b>
 </ul>
 
 <h2><a name="unix_access">Unix machine access</a></h2>