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: 6bb63ccc ("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.netSigned-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/mptcp-sysctl.rst

@@ -47,7 +47,6 @@ allow_join_initial_addr_port - BOOLEAN
 	Default: 1
 
 pm_type - INTEGER
-
 	Set the default path manager type to use for each new MPTCP
 	socket. In-kernel path management will control subflow
 	connections and address advertisements according to