mptcp: Fix grammar in a comment

We kept getting initial patches from new contributors to remove a
duplicate 'the' (since grammar checking scripts flag it), but submitters
never followed up after code review.

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>