virtio_blk: Fix English description of VIRTIO_BLK_SCSI

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 97b50a65 ("virtio_blk: make SCSI passthrough support configurable")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>