Improve Jupyter-lab setup instructions

[Ben Hampson]

In the setup, the first two lines are:

virtualenv -p python 3.8 venv-3.8-jupyter
source ~/venv-3.8-jupyter/bin/activate

1. I think it should be python3.8 not python 3.8
2. I think it would be better to be source venv-3.8-jupyter/bin/activate without the ~/, seeing as most people probably won't set up the virtualenv in ~/

[Ben Hampson]

I will take this on!