Fix a small typo when creating a Herald rule

Summary: "Users who an edit" to "Users who can edit"

Test Plan: Verified that typo is gone after the change

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: aran, epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8511