Corrected Expat explanation on Rationale page since systemd isn't in the book

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@10615 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

prologue/why.xml

@@ -151,7 +151,7 @@
         <para>Expat</para>
 
         <para>This package contains programs for parsing XML.  It is
-        required for systemd.
+        required for XML-Parser.
         </para>
       </listitem>