block, documentation: Fix wbt_lat_usec documentation

Fix the spelling of the wbt_lat_usec sysfs attribute.

Fixes: 87760e5e ("block: hook up writeback throttling") # v4.10.
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>