Documentation: block: blk-mq: Fix small typo in multi-queue docs

Fixed a small typo in the documentation for multi-queue block IO.

Signed-off-by: Jinay Jain <jinaybjain@gmail.com>
Link: https://lore.kernel.org/r/20210812152528.300668-1-jinaybjain@gmail.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>