MAINTAINERS: update DCO info

Drop the URL for DCO (URL is invalid).  Also, point to SubmittingPatches
for the current DCO.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>