docs: zh_CN: howto.rst: fix a broken reference

There's a broken reference there pointing to the long gone
DocBook dir.

While I don't read chinese, Google translator translates it
to:
	"The generated documentation will be placed in
	 the Documentation/DocBook/ directory."

Well, we know that the output will be Documentation/output
dir. So, let's fix this one.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>