Add a label to the last exercise in logic.tex

This way we can use it in HoTT/HoTT
Add exercise about finite choice

I'm not sure if I should include a note about the exercise requiring
functional extensionality, or about the exercise not requiring the
assumptions that A(x) are sets and P(x, a) are mere propositions.  I'm
also not sure if I should be marking an erratum for this.

This closes #622.
Merge pull request #624 from JasonGross/finite-choice

Finite choice