Update Dask extension instructions

[troyraen]

It appears to me that our instructions for using the Dask extension (#52) are out of date. That, or else I'm doing something wrong but don't see it yet. I'm running into the issue described here:

According to the Quick-start instructions in this section of the Fornax user guide, I should be able to start a dask cluster from the Jupyter extension, skip this line of code, and then be able to monitor the workers, etc. from the extension's charts. But it isn't working for me. When I follow the instructions and comment out this cell, I hit an error the first time dask workers actually need to do something (euclid_stars.head(10)):

...
UnpicklingError: state is not a dictionary
The above exception was the direct cause of the following exception:
RuntimeError                              Traceback (most recent call last)
...
RuntimeError: Error during deserialization of the task graph. This frequently
occurs if the Scheduler and Client have different environments.
For more information, see
https://docs.dask.org/en/stable/deployment-considerations.html#consistent-software-environments

Originally posted by @troyraen in Caltech-IPAC/irsa-tutorials#73 (comment)