doc: Fix typo in request queue sysfs documentation

Fix a typo (are -> as) in the introduction paragraph of
Documentation/block/queue-sysfs.rst.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20211027022223.183838-6-damien.lemoal@wdc.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>