Fix wrong plural of an arc land message

Summary:
"Branch" was pluralized as "branchs".

Fixes T9225.

Test Plan:

• Created test repo with two revisions on a feature branch.
• Saw old message, frowned a little.
• Applied patch.
• No longer frowning.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: avivey, Korvin

Maniphest Tasks: T9225

Differential Revision: https://secure.phabricator.com/D13944