Evaluation metrics in model selection lesson

[agitter]

In the model selection lesson, we can list the evaluation metrics when in the "Evaluation Metrics" section and state that we'll define and explore each of them in more detail later in the lesson. We can also move the common metrics before the scenario.

We can add text that captures what was discussed during the lesson about many true negatives leading to high accuracy in settings where high accuracy may not be important or meaningful.