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!
Aha! After rechecking, I see that merely enclosing the letters works just fine here as well. I wrongly assumed that it did not, since it was wrong in the linked PDF and I didn't consider the possibility that the repository already contained a fixed version.

Sorry for the noise. I'll add a further commit which restores the original version. If wanted, I can squash the commits into one (either as a new pull request, or as a forced-update to this one. But I'm told that in this case our discussion gets lost).