Book 2.8: Instances introduction needs improvement

[hollasch]

The end of the introductory part of the Instances section — the paragraph beginning with "Now that we have boxes, …" — needs some rework. Besides rewording, the sentence "This is especially easy …" should start its own paragraph, probably preceded by the figure.

Further, the figure is wrong, as it shows both the ray and the box moving in the same way, instead of the ray undergoing an inverse transformation. We could use a better figure for this.