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: 1ce4da3 ("docs: use parser_yaml extension to handle Netlink specs")
Reported-by: Kory Maincent <[email protected]>
Closes: https://lore.kernel.org/20250828185037.07873d04@kmaincent-XPS-13-7390
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: matttbe

Documentation/networking/mptcp.rst

@@ -66,7 +66,7 @@ same rules are applied for all the connections (see: ``ip mptcp``) ; and the
 userspace one (type ``1``), controlled by a userspace daemon (i.e. `mptcpd
 <https://mptcpd.mptcp.dev/>`_) where different rules can be applied for each
 connection. The path managers can be controlled via a Netlink API; see
-netlink_spec/mptcp_pm.rst.
+../netlink/specs/mptcp_pm.rst.
 
 To be able to use multiple IP addresses on a host to create multiple *subflows*
 (paths), the default in-kernel MPTCP path-manager needs to know which IP