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

Documentation/usb/usbmon.rst

@@ -42,7 +42,7 @@ if usbmon is built into the kernel::
 	# modprobe usbmon
 	#
 
-Verify that bus sockets are present:
+Verify that bus sockets are present::
 
 	# ls /sys/kernel/debug/usb/usbmon
 	0s  0u  1s  1t  1u  2s  2t  2u  3s  3t  3u  4s  4t  4u