docs: block: blk-mq.rst: correct places -> place

Corrected a typo in the blk-mq.rst documentation where 'places' was
incorrectly used instead of 'place'.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20230902164121.2653109-1-visitorckw@gmail.com>