Fix typo in injectives.tex

Thanks to Minseon Shin
Typo in proetale.tex

Thanks to Keenan Kidwell
http://stacks.math.columbia.edu/tag/0988#comment-497
Improve the counter example to "Deligne's formula"

This is a copy of the answer

http://math.stackexchange.com/a/731437/120019

to a question of Martin Brandenburg.