fix reference in proof of 4.7.6, close #356.

I had to be careful not to alter the numbering of anything else on
this one, so whoever merges it should check that I didn't mess up.
Merge branch 'master' into sumoverpaths
add 3d119cf to errata
Merge branch 'sumoverpaths' for publication, as instructed by @andrejbauer

Conflicts:
	errata.tex