usb:gadget Fix comment for pointer to configfs

The documentation for the USB gadget fs is actually in
Documentation/usb/gadget_configfs.txt.
Signed-off-by: NPhilippe De Swert <philippe.deswert@jollamobile.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/Kconfig

@@ -555,7 +555,7 @@ config USB_CONFIGFS
 	  specified simply by creating appropriate directories in configfs.
 	  Associating functions with configurations is done by creating
 	  appropriate symbolic links.
-	  For more information see Documentation/usb/gadget-configfs.txt.
+	  For more information see Documentation/usb/gadget_configfs.txt.
 
 config USB_CONFIGFS_SERIAL
 	boolean "Generic serial bulk in/out"