zh_CN: missed translation of systemd-umount desc

chapter06/systemd.xml

@@ -694,7 +694,7 @@ meson --prefix=/usr                \
       <varlistentry id="systemd-umount">
         <term><command>systemd-umount</command></term>
         <listitem>
-          <para>Unmount mount points</para>
+          <para>卸载挂载点</para>
           <indexterm zone="ch-system-systemd systemd-umount">
             <primary sortas="b-systemd-umount">systemd-umount</primary>
           </indexterm>