Doc: ABI: testing: configfs-usb-gadget-sourcesink

Fix the name of attribute

Cc: <stable@vger.kernel.org>
Signed-off-by: NPeter Chen <peter.chen@freescale.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

Documentation/ABI/testing/configfs-usb-gadget-sourcesink

@@ -9,4 +9,4 @@ Description:
 		isoc_maxpacket	- 0 - 1023 (fs), 0 - 1024 (hs/ss)
 		isoc_mult	- 0..2 (hs/ss only)
 		isoc_maxburst	- 0..15 (ss only)
-		qlen		- buffer length
+		buflen		- buffer length