Fix broken experimenter repo links to app

[mikewilli]

This section links to https://github.com/mozilla/experimenter/blob/main/app/experimenter/targeting/constants.py, which should be https://github.com/mozilla/experimenter/blob/main/experimenter/experimenter/targeting/constants.py.

We should fix this link and any other similar links that point to the renamed "app" folder in the Experimenter repo.