Tutorial 4: objective function example revision

[Hwei-Shin Harriman]

Link to example: https://penrose.gitbook.io/penrose/tutorial-4-writing-constraints-and-objectives#objectives-example-circle-repel
The provided objective function example does not have a local minima, unlike the description suggests that it should. Consider switching to another objective function example that contains a local minima and updating the tutorial as necessary.