Clarify instructions for repository remote uri

Summary:
Clarified the instructions when editing a repository for the remote uri to
mention that a local path for the remote uri must be specified as
file:///local/path/to/repo instead of /local/path/to/repo. (The latter used to
work, but stopped functioning for new repositories as of D888.)

Test Plan:
loaded /repository/edit/NN/tracking (where NN is a repo number), and saw the
updated instruction message.

Reviewers: jungejason, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: 1005