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!
Hmm, I might rather enclose it in `\url` then; in general I don't like writing URLs in variable-width fonts.