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'd like to hear some other opinions about url formatting.  If yours is in
the majority I'll happily concede.

On Thu, Dec 25, 2014 at 2:12 AM, Ingo Blechschmidt <notifications@github.com
> wrote:
>
> In front.tex
> <https://github.com/HoTT/book/pull/751#discussion-diff-22271031>:
>
> > @@ -94,12 +94,12 @@
> >  \textbf{\emph{Creative Commons Attribution-ShareAlike 3.0 Unported License.}}
> >  %
> >  To view a copy of this license, visit
> > -\href{http://creativecommons.org/licenses/by-sa/3.0/}{http://creativecommons.org/licenses/by-sa/3.0/}.
> > +\href{http://creativecommons.org/licenses/by-sa/3.0/}{http:/\!/creativecommons.org/licenses/by-sa/3.0/}.
>
> Then quite a few \href's have to be changed to \url's. I can do this, if
> you want.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/HoTT/book/pull/751/files#r22271031>.
>