docs/zh_CN: fix rst format errors in howto.rst

Tame howto.rst file to avoid "ERROR: Unexpected indentation." etc
compiler errors.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Harry Wei <harryxiyou@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Li Yang <leoyang.li@nxp.com>
Cc: TripleX Chung <xxx.phy@gmail.com>
Signed-off-by: Weiwei Jia <harryxiyou@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>