Use latest maven-sdocbook-plugin

doc/xdocs/reference.xml

@@ -54,7 +54,7 @@
 
           <tr>
             <td>
-              <a href="docbook/index.pdf">Reference Guide PDF</a>
+              <a href="docbook/acegi.pdf">Reference Guide PDF</a>
             </td>
             <td>
               The PDF version of the reference guide.