docs: usb: fix reference to nonexistent file in UVC Gadget

Fix a typo in the path of this reference.

Fixes: 094f391013ba ("docs: usb: Add documentation for the UVC Gadget")
Cc: Daniel Scally <dan.scally@ideasonboard.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20231022185311.919325-1-vegard.nossum@oracle.com>