Litespeed: Add reference to new book (#289)

* Litespeed: Add reference to new book

Author: jordikroon

manual.xml

@@ -346,6 +346,7 @@
   <set xml:id="refs.utilspec.server">
    <title>&ServerSpecificExtensions;</title>
    &reference.apache.book;
+   &reference.litespeed.book;
    &reference.fpm.book;
   </set>