Fix Passphrase cancel URI

Summary: The "Cancel" button on the "Edit Credential" interface doesn't go back to the "View Credential" interface for existing credentials.

Test Plan: Clicked "Cancel" on both "create" and "edit" workflows.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D8568