Fix typos in README

[paultcochrane]

While reading the README I spotted a couple of typos which I thought you'd like to have fixed. I also spotted an extra comma and some trailing whitespace which you also might not like to have.

I've split up the commits so that you can cherry pick them as you wish. If you want, I could squash the commits into one, or I could remove any commits you don't see as necessary and then resubmit the PR. Either way, this PR is submitted in the hope that it is useful, so if you have any comments or wish for any changes, please don't hesitate to contact me :-)