Fix link to mrbgems documentation.

Author: oppegard

libraries/index.html

@@ -11,7 +11,7 @@ <h2 class='page-header'>Libraries</h2>
 <div class='row'>
   <div class='col-md-12'>
     <p>
-      <strong>mrbgems</strong> is mruby's package manager. See the <a href="https://github.com/mruby/mruby/tree/master/doc/mrbgems">mrbgems README</a> for more information about how to use it.
+      <strong>mrbgems</strong> is mruby's package manager. See the <a href="https://github.com/mruby/mruby/blob/master/doc/guides/mrbgems.md">mrbgems doc</a> for more information about how to use it.
     </p>
   </div>
 </div>