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 didn't mean that putting braces around single letters doesn't suffice to make them capital; it certainly does for me.  (It doesn't for you?)  I've just read that it makes the spacing or something else slightly off.