Adding trailing slash to 'Edit Mock' URI

Summary: This one kind of annoyed me when debugging for D5345, since I have stop-on-redirect to on. I know that I have missed showtime, but still, better late than never

Test Plan: Verified that I was not stopped when editing a mock

Reviewers: epriestley, chad, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5388