Documentation/process: fix a cross reference

The cross-reference for the handbooks section works. However, it is
meant to describe the path inside the Kernel's doc where the section
is, but there's an space instead of a dash, plus it lacks the .rst at
the end, which makes:

	./scripts/documentation-file-ref-check

to complain.

Fixes: 604370e1

 ("Documentation/process: Add maintainer handbooks section")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>