Fixed several tiny (non-mathematical) typos

This pull request corrects several tiny typos. They are all non-mathematical, so probably do not need an entry in the errata list.

I added a few non-breaking spaces, only in places where it looked really necessary to do so in my particular printing (A4).

Merry Christmas!
I think this one is fine, except that maybe it should take the subjunctive instead: "we would rather that a given function _could_ ``be an equivalence'' in at most one way".