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!
Indeed, enclosing only the single letters doesn't suffice with my LaTeX installation (and, apparently, with the one used to build http://hottheory.files.wordpress.com/2013/03/hott-online-611-ga1a258c.pdf). In the next commit, I'll enclose "Martin-Löf".