scsi: core: Update SCSI_MQ_DEFAULT help text to match default

The default was changed, but the help text was not updated.

Fix grammar (s/the option/this option/) while at it.

[mkp: drop "new" as suggested by John Garry]

Fixes: d5038a13 ("scsi: core: switch to scsi-mq by default")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>