update copyright year to 2020 in CLI usage output
update copyright year to 2020 in CLI usage output

Summary: I was running watchman -h and saw this said 2017, so updated to 2020

Reviewed By: chadaustin

Differential Revision: D19320425

fbshipit-source-id: ad26d74edfcca0d38c9f6614016b16e5104351e0

facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman