Htmldocs does not display the chapter number, convert the two manual
chapter number tags to ref tags.

Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Link: https://lore.kernel.org/r/20210403113752.GA32236@mipc


Signed-off-by: Jonathan Corbet <corbet@lwn.net>