Update sample code after deprecating `getConfiguration` in favor of `StartConfigurationSession`

Hi 👋,
We are currently evaluating `AppConfig`.

The apis used in this sample code have been deprecated in favor of [StartConfigurationSession](https://docs.aws.amazon.com/appconfig/2019-10-09/APIReference/API_appconfigdata_StartConfigurationSession.html)

I can't find any resources on how to programmatically use `StartConfigurationSession`.