Can you paste your URL into a browser and get the file you expect? I suspect the file is not publicly available. Instead, I would suggest something like this to restore the latest staging backup:
heroku pgbackups:restore DATABASE -a example-staging `heroku pgbackups:url -a example`
or this to restore a specific older backup:
heroku pgbackups:restore DATABASE -a example-staging `heroku pgbackups:url a114 -a example`