cdrom: update ioctl documentation

Correct copy-paste problem: CDROMCLOSETRAY is about closing the tray,
not opening it.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>