Fix minor section formatting mishap in SSH key email

Summary: Ref T10917. This is getting added as a link right now, which causes it to get <a href>'d in HTML mail. Add it as text instead.

Test Plan: Edited a key, examined HTML mail body carefully.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10917

Differential Revision: https://secure.phabricator.com/D15952