Use new naming rules for config.json concepts

We've recently introduced rules regarding the [naming of concepts](https://github.com/exercism/v3/blob/master/docs/maintainers/determining-concepts.md#naming-concepts) in `config.json` files.

The Dart track's [config.json](https://github.com/exercism/v3/blob/master/languages/dart/config.json) file should be updated to conform to the new naming rules.