Documentation: fix usb related doc refs

Update ref to usb proc_usb_info.txt.

Signed-off-by: Tom Saeger <tom.saeger@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/usb/gadget-testing.txt

@@ -773,7 +773,7 @@ host:
 # cat /dev/usb/lp0
 
 More advanced testing can be done with the prn_example
-described in Documentation/usb/gadget-printer.txt.
+described in Documentation/usb/gadget_printer.txt.
 
 
 20. UAC1 function (virtual ALSA card, using u_audio API)