python http module only available in Python 3

[alexpearce]

In the "contributing" lesson, we say users can start a local web server with

$ python -m http.server

Python 2 doesn't have the http module. The equivalent would be

$ python -m SimpleHTTPServer

We could either add a callout, or maybe just specify the Python version in the command, e.g.

$ python2 -m SimpleHTTPServer