EmrEtlRunner: make format options more explicit in config example

[jbeemster]

The config sample can be a bit misleading when it comes to selecting "CloudFront" as your target - it is there by default but it is not explained what it processes as such.

In can be easy to associate the CloudFront collector with CloudFront access logs in this scenario and not be able to process any data - see this thread for context:

https://discourse.snowplowanalytics.com/t/app-id-is-null-even-tough-aid-is-set-in-cs-uri-query/2736/3