Using those fields that have an underscore (instead of a hyphen) cause
the following warnings if used:
```
Warning: Input 'app_id' has been deprecated with message: 'app_id' is deprecated and will be removed in a future version. Use 'app-id' instead.
Warning: Input 'private_key' has been deprecated with message: 'private_key' is deprecated and will be removed in a future version. Use 'private-key' instead.
```
So this PR just drops the last use of `app_id` and `private_key` from
the README in favor of `app-id` and `private-key`.
