Update doc on installation for JupyterLab for python3

[stefaneidelloth]

If I follow the outdated instruction

https://github.com/quantopian/qgrid#jupyterlab-installation

that says

jupyter labextension install qgrid2

... the widget is not displayed. Also see https://github.com/quantopian/qgrid/issues/336

If I use following command in addition, it works:

jupyter labextension install qgrid

(However, when starting JupyterLab I get a message

"Plugin 'qgrid' is already registered"

=> Please clarify what steps are required for the installation and update the documentation.

If I only use

jupyter labextension install qgrid

the widget does not work. So maybe qgrid needs qgrid2 as a dependency. Looks like a bug to me.