docs/ABI: use linux-s390 list as the main contact

Remove Cornelia's email address from the file as suggested by her. List
linux-s390 mailing-list address as the primary contact instead.

Link: https://lore.kernel.org/linux-s390/8735d0oiq6.fsf@redhat.com/


Signed-off-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>