Update jupyterlab instructions in readme

[marius311]

I've never used this package before, but on my first attempt following the instructions in the readme I got this error with Jupyterlab 2.2.4

$ jupyter labextension install @jupyterlab/plotly-extension
An error occured.
ValueError: The extension "@jupyterlab/plotly-extension" does not yet support the current version of JupyterLab.


Conflicting Dependencies:
JupyterLab                        Extension      Package
>=2.2.0 <2.3.0                    >=1.3.0 <2.0.0 @jupyterlab/rendermime-interfaces
See the log file for details:  /tmp/jupyterlab-debug-tso8ar1l.log

Reading that logfile hinted that the correct solution seems to be

$ jupyter labextension install jupyterlab-plotly

If so, would be good to update the readme.

[sglyon]

Thank you for reporting. I will make the update!