docs: block: blk-mq.rst: correct drive -> driver

It is 'driver' to complete the request. Also remove a redundant space.

Signed-off-by: Yue Hu <huyue2@yulong.com>
Link: https://lore.kernel.org/r/20210520074225.1989-1-zbestahu@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>