Documentation for Contributing.md - Starting Virtual Environment Command Line Not Working

Hi,

I noticed that the instructions on starting a virtual environment is not working and is different from what is shown in the youtube videos that were created to illustrate the set up process. Specifically this line of code:

$ virtualenv --python=python3 env

This code gives me an error when I run it in my terminal and a workaround was to use this command.

python3 -m venv env

