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

Fix the name of attribute

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

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

@@ -5,4 +5,4 @@ Description:
 		The attributes:
 
 		qlen		- depth of loopback queue
-		bulk_buflen	- buffer length
+		buflen		- buffer length