Correct "Lights" paragraph in "What's next?" list, adjust numbering.

[shaunplee]

In the "What's next?" list in Chapter 13, it seems like the paragraphs:

1. Lights. You can do this explicitly, by sending shadow rays to lights. Or it can be done implicitly by making some objects emit light,
2. Biasing scattered rays toward them, and then downweighting those rays to cancel out the bias. Both work. I am in the minority in favoring the latter approach.

should actually be part of the same paragraph. Combining these paragraphs also means renumbering the remaining paragraphs.

[trevordblack]

Please create a separate issue.

Also, as far as "What's Next" we're in the middle of figuring out how we want to organize all of them together. See #407

If you have any ideas, feel free to throw them in there

[shaunplee]

Ok. New issue opened #472

[hollasch]

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