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

commit 4bc58eb1

 upstream.

Fix the name of attribute

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>