chapter06: Fix expat instructions

Approved by upstream:
<http://lists.linuxfromscratch.org/pipermail/lfs-dev/2018-April/072217.html>

chapter06/expat.xml

@@ -46,7 +46,9 @@
 
     <para>准备安装 Expat：</para>
 
-<screen><userinput remap="configure">./configure --prefix=/usr --disable-static</userinput></screen>
+<screen><userinput remap="configure">./configure --prefix=/usr \
+            --disable-static \
+            --docdir=/usr/share/doc/expat-&expat-version;</userinput></screen>
 
     <para>编译该软件包：</para>