python http module only available in Python 3

In the "contributing" lesson, we say users can start a local web server with

```shell
$ python -m http.server
```

Python 2 doesn't have the `http` module. The equivalent would be

```shell
$ python -m SimpleHTTPServer
```

We could either add a callout, or maybe just specify the Python version in the command, e.g.

```shell
$ python2 -m SimpleHTTPServer
```