minor typo fixes

Thanks for all this detail, I've found your docker posts very interesting also. Haven't yet found a compelling reason to use docker or BCE in my workflow, mostly due to the need to work with students and keep the toolkit as small and simple as possible. I just have [a shell script](https://gist.github.com/benmarwick/11204658) that provisions my VM (which might be on a PC, mac or linux machine) with stuff I need, and call that my environment. What am I missing by not using these other en vogue tools? Since I can't assume a linux machine (due to students), docker doesn't seem to add anything. And my provisioning script is so simple I'm not sure if one of the other tools would save much effort.
Merge pull request #120 from benmarwick/patch-1

minor typo fixes