Change man-pages maintainer address

The email address that I use for man-pages maintenance has changed as of
now.  This patch changes the address in Documentation/HOWTO and
MAINTAINERS.

Signed-off-by: Michael Kerrisk <mtk.manpages@googlemail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>