Changed the main page u4x link to link to the Book

source/index.html

@@ -53,7 +53,7 @@ <h4><a href="/compilers/">Compilers</a></h4>
     <div id="docs" class="box">
         <h4><a href="/documentation/">Documentation</a></h4>
         <ul>
-        <li><a href="http://github.com/perl6/mu/tree/master/docs/u4x/">u4x</a>, User-level documentation for Christmas</li>
+        <li><a href="http://github.com/perl6/book/downloads">Using Perl 6</a>, an open-source Perl 6 book</li>
         <li><a href="http://perlgeek.de/en/article/5-to-6">Perl&nbsp;5 to 6</a>,
             introduction for Perl&nbsp;5 programmers</li>
         </ul>