Documentation: dynamic-debug-howto: fix example

dynamic debug is "expecting pairs of match-spec <value>" so the example
for all files of which the paths include "usb" there is "file" missing.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Link: https://lore.kernel.org/r/20210303091646.773111-1-martin.kepplinger@puri.sm


Signed-off-by: Jonathan Corbet <corbet@lwn.net>