docs: usb: function: Modify path name

Original path does not exists, so changed to
"Documentation/ABI/testing/configfs-usb-gadget"

Signed-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Link: https://lore.kernel.org/r/20210506122020.7117-1-jj251510319013@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>