MAINTAINERS: mptcp@ mailing list is moderated

Note that mptcp@lists.01.org is moderated, like we note for
other mailing lists.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Mat Martineau <mathew.j.martineau@linux.intel.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Cc: netdev@vger.kernel.org
Cc: mptcp@lists.01.org
Signed-off-by: David S. Miller <davem@davemloft.net>