USB: fix paths in usbmon documentation

Hi there.

On Aug 21 2009, Alan Stern wrote:
> On Thu, 20 Aug 2009, Rogério Brito wrote:
> > Again, just reiterating, what I said before, even though I am not sure
> > if I can reproduce it, I will try to.
> 
> A usbmon trace showing what happens when you plug in the drive and 
> when you run smartctl would help.

The documentation for usbmon in the kernel 2.6.31-rc7 kernel doesn't
match what the kernel exposes in the debug fs tree. This patch fixes it.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>