doc: mptcp: fix Netlink specs link

doc: mptcp: fix Netlink specs link

The Netlink specs RST files are no longer generated inside the source
tree.

In other words, the path to mptcp_pm.rst has changed, and needs to be
updated to the new location.

Fixes: 1ce4da3dd99e ("docs: use parser_yaml extension to handle Netlink specs")
Reported-by: Kory Maincent <kory.maincent@bootlin.com>
Closes: https://lore.kernel.org/20250828185037.07873d04@kmaincent-XPS-13-7390
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250909-net-next-mptcp-pm-link-v1-1-0f1c4b8439c6@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>