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 cur_win_nsec member of struct
rq_wb.

Signed-off-by: Tang Yizhou <[email protected]>

Author: Tang Yizhou

Documentation/ABI/stable/sysfs-block

@@ -731,11 +731,11 @@ Contact:	[email protected]
 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
-		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
-		setting.
+		is exceeded in a given window of time (see the cur_win_nsec
+		member of struct rq_wb), 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 setting.
 
 
 What:		/sys/block/<disk>/queue/write_cache