usb: fix the comment with regards to DocBook

The USB gadget documentation is not at DocBook anymore.
The main file was converted to ReST, and stored at
Documentation/driver-api/usb/gadget.rst, but there are
still several plain text files related to gadget under
Documentation/usb.

So, be generic and just mention documentation
without specifying where it is.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/usb/gadget/Kconfig

@@ -41,7 +41,7 @@ menuconfig USB_GADGET
 	   don't have this kind of hardware (except maybe inside Linux PDAs).
 
 	   For more information, see <http://www.linux-usb.org/gadget> and
-	   the kernel DocBook documentation for this API.
+	   the kernel documentation for this API.
 
 if USB_GADGET