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!
Weird!  What is your installation?

The file you link to was built before commit 275d6360 which added the
braces around single letters.

On Thu, Dec 25, 2014 at 2:19 AM, Ingo Blechschmidt <notifications@github.com
> wrote:
>
> In references.bib
> <https://github.com/HoTT/book/pull/751#discussion-diff-22271153>:
>
> > @@ -445,7 +445,7 @@ @article{hedberg1998coherence
> >  	Number = {4},
> >  	Pages = {413--436},
> >  	Publisher = {Cambridge Univ Press},
> > -	Title = {A coherence theorem for {M}artin-{L\"o}f's type theory},
> > +	Title = {{A coherence theorem for Martin-L\"of's type theory}},
>
> 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".
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/HoTT/book/pull/751/files#r22271153>.
>