MAINTAINERS: fix location of moxa-smartio.rst

MAINTAINERS: fix location of moxa-smartio.rst

The moxa-smartio.rst file was moved to a different location,
so update the MAINTAINERS file entry for it.

Quietens one build warning:

MAINTAINERS:29715: WARNING: unknown document: ../driver-api/serial/moxa-smartio

Fixes: 7f0dd8bf531f ("Documentation: tty: move moxa-smartio.rst to tty")
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220420235545.17081-1-rdunlap@infradead.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>