Update arcanist NOTICE file

Summary: See D9730.

Test Plan: reading

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9731
Patch: don't complain about reviewers not being specified.
This will break in the future when upstream arc incorporates T4631. Hopefully there will be a better solution by then