Documentation/process/howto: Mark subsection in suggested format

`Specific guidelines for the kernel documentation` section of
`kernel-documentation.rst` suggests to use ``~`` for subsection but
subsections in HOWTO is not marked in the format.  This commit marks
them in the format.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>