Typo in resolve transfer method

Before this change, when running `pgbackups:transfer` from a database in one application to a database in another application I see:

```
$ heroku pgbackups:transfer DATABASE_URL postgres://u:p@server.amazonaws.com:db -a app
"https://s3.amazonaws.com/hkpgbackups/app@heroku.com/b123.dump?AWSAccessKeyId=k&Expires=1390504988&Signature=s"

 !    invalid format
```

With the fix it works fine, because the `to_url` in the `transfer` method gets set to the passed argument `url`.