block, documentation: Fix wbt_lat_usec documentation

Fix the spelling of the wbt_lat_usec sysfs attribute.

Fixes: 87760e5 ("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>

Documentation/block/queue-sysfs.txt

@@ -185,8 +185,8 @@ This is the number of bytes the device can write in a single write-same
 command.  A value of '0' means write-same is not supported by this
 device.
 
-wb_lat_usec (RW)
-----------------
+wbt_lat_usec (RW)
+-----------------
 If the device is registered for writeback throttling, then this file shows
 the target minimum read latency. If this latency is exceeded in a given
 window of time (see wb_window_usec), then the writeback throttling will start