Typos fix

I added some missing underscores.

The `make docs` command had no effect, so I generated the html file by hand. I kept that in a separate commit as it seems it changes more HTML chunks than I'd expect, just in case.
I don't have `pdflatex` available on this machine, so I didn't update the pdf file.

If the last commit is not welcome, let me know and I'll remove it.