Update path in startup error

Summary: The error mentioned a "phabricator" directory, but according to the docs it has been migrated to "phorge".

Test Plan: Move the "arcanist" directory somewhere else, run something from phorge/bin/.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25445