docs: usb: fix literal block marker in usbmon verification example

The "Verify that bus sockets are present" example was not properly
formatted due to a typo in the literal block marker.

Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Link: https://lore.kernel.org/r/20220604155431.23246-1-justin.swartz@risingedge.co.za


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