[SCSI] scsi_debug: correct parameter default text

Correct the module info text for the default value of
"every_nth" to 0.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>