Fix broken link in phabricator documentation

Summary:
The link to libphutil Libraries User Guide was broken on Arcanist User
Guide: Customizing Lint, Unit Tests and Workflows.

Test Plan: ran diviner

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1890