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!
What's the rationale for this?  I've read that it's better to put the braces around the whole word, e.g. `{Martin-L\"of}`, but why the whole title?