Documentation: mptcp: fix pm_type formatting

Documentation: mptcp: fix pm_type formatting

When looking at the rendered HTML version, we can see 'pm_type' is not
displayed with a bold font:

  https://docs.kernel.org/5.19/networking/mptcp-sysctl.html

The empty line under 'pm_type' is then removed to have the same style as
the others.

Fixes: 4e7d48005f8d ("mptcp: Add a per-namespace sysctl to set the default path manager type")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20220906180404.1255873-2-matthieu.baerts@tessares.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>