Projects - fix translation strings in watcher edge class

Summary: Fixes T7319. These need a "%s" for the count where they had a "%d"

Test Plan: plan in D11812 is no longer a lie! (watcher added / removed strings render correctly)

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7319

Differential Revision: https://secure.phabricator.com/D11813