removed an unnecessary restriction in exercise 7.5
forgot part of the change
linebreak added
made the base case clear
cite Hofmann and Streicher for univalence and categories (pointed out by Benedikt)
improved phrasing, more detail
Merge pull request #522 from mikeshulman/citehs

cite Hofmann and Streicher for univalence and categories
eliminate premature "Theorem", see #519
add exercise from Marc, close #498
marked errata for ga57eab2
mark errata for 39dfbd8
unmarked errata for 810aa94
Merge pull request #518 from sattlerc/master

removed an unnecessary restriction in exercise 7.5
Merge pull request #526 from mikeshulman/lem2.10.5ex

add exercise from Marc, close #498
Merge pull request #523 from mikeshulman/firsttime

eliminate premature "Theorem", see #519
marked errata for 810aa94