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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/usb/gadget_configfs.rst

@@ -140,7 +140,7 @@ is an arbitrary string allowed in a filesystem, e.g.::
 Each function provides its specific set of attributes, with either read-only
 or read-write access. Where applicable they need to be written to as
 appropriate.
-Please refer to Documentation/ABI/*/configfs-usb-gadget* for more information.
+Please refer to Documentation/ABI/testing/configfs-usb-gadget for more information.
 
 4. Associating the functions with their configurations
 ------------------------------------------------------