wording/formatting in ch 10
Fix well-founded induction.

On re-reading, I couldn't make any sense of the proof of 10.3.2.  In
order to fix it, I found I needed to write out the induction principle
for accessibility more carefully, which is probably a good idea anyway.
Fix proof of union axiom for V

I don't think f\circ\pi was the right map.
Merge branch 'master' of github.com:HoTT/book