MAINTAINERS: correct rmk's email address

Correct my email address in the MAINTAINTERS file.

Link: http://lkml.kernel.org/r/E1ezkgi-0002fH-01@rmk-PC.armlinux.org.uk
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>