Update link to the book

Refs: libevent/libevent-book#1

Author: azat

index.html

@@ -122,7 +122,7 @@ <h3>Sponsors</h3>
 <tr>
 <td valign="top" id="download">
 <h3>Documentation</h3>
-Book: <a href="https://libevent.org/book/">Programming with Libevent</a>.<br>
+Book: <a href="https://libevent.org/libevent-book/">Programming with Libevent</a>.<br>
 Reference:
 <a href="http://www.wangafu.net/~nickm/libevent-1.4/doxygen/html">1.4.x-stable</a>
 <a href="http://www.wangafu.net/~nickm/libevent-2.0/doxygen/html">2.0.x-stable</a>