Fix missing comma in sprintf() in ArcanistWorkflow.

Summary: Refs D12607. Fixes T8195. Replace period in the sprintf() arguments with a comma.

Test Plan: Ran arc diff in the patch applied, did not get the sprintf() error

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T8195

Differential Revision: https://secure.phabricator.com/D12837