Correct two xrefs which now mislead users to wrong chapters due to

advanced-networking chapter split.

Approved by:	doceng (hrs)

en_US.ISO8859-1/books/handbook/mail/chapter.sgml

@@ -114,7 +114,7 @@
 
       <listitem>
 	<para>Properly set up the DNS information for your mail host
-	  (<xref linkend="advanced-networking">).</para>
+	  (<xref linkend="network-servers">).</para>
       </listitem>
 
       <listitem>

en_US.ISO8859-1/books/handbook/users/chapter.sgml

@@ -667,7 +667,7 @@ Other information:</screen>
 	  &man.ypchsh.1;.  NIS support is automatic, so
 	  specifying the <literal>yp</literal> before the command is
 	  not necessary.  If this is confusing to you, do not worry, NIS will
-	  be covered in <xref linkend="advanced-networking">.</para>
+	  be covered in <xref linkend="network-servers">.</para>
       </note>
     </sect2>
     <sect2 id="users-passwd">