blk-wbt: doc: Update the doc of the wbt_lat_usec interface

The symbol wb_window_usec cannot be found. Update the doc to reflect the
latest implementation, in other words, the debugfs interface
'curr_win_nsec'.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>

Author: Tang Yizhou

Documentation/ABI/stable/sysfs-block

@@ -731,7 +731,7 @@ Contact:	linux-block@vger.kernel.org
 Description:
 		[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
+		is exceeded in a given window of time (see curr_win_nsec), then
 		the writeback throttling will start scaling back writes. Writing
 		a value of '0' to this file disables the feature. Writing a
 		value of '-1' to this file resets the value to the default