Typo in Using pip and virtualenv training mission

[ghost]

It's on: https://openhatch.org/missions/pipvirtualenv/reference
"To activate a virtualenv that has already been created, type workon followed by the name of the virtualenv that you wish to activate, like so:
mkvirtualenv TestEnv"

[doug-wade]

I could fix this, and submit a pull request, but I think this is a great problem for someone new who wants to contribute a fix. The typo, iiuc, is in this file, which is the template that generates this mission's quick reference. As @t-darxt noticed, we should be using workon rather than mkvirtualenv, and the example env is FooEnv, not TestEnv. If you look at the docs, it should be clear what the change to be made is (hint: the relevant part starts with "Of course we are not limited to a single virtualenv").