CREDITS: Update email address for Mat Martineau

Changing my email address in CREDITS to be consistent with what's in use
in MAINTAINERS and mailmap. Also removed extra date information from the
CREDITS entry since I'm a maintainer for MPTCP again.

Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230502000545.2899055-1-martineau@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: mjmartineau

CREDITS

@@ -2510,8 +2510,8 @@ D: XF86_8514
 D: cfdisk (curses based disk partitioning program)
 
 N: Mat Martineau
-E: mat@martineau.name
-D: MPTCP subsystem co-maintainer 2020-2023
+E: martineau@kernel.org
+D: MPTCP subsystem co-maintainer
 D: Keyctl restricted keyring and Diffie-Hellman UAPI
 D: Bluetooth L2CAP ERTM mode and AMP
 S: USA