emr-notebooks-magics example install command includes very dubious pip options

I spotted this while doing some work with EMR and looking at the example notebooks:

https://github.com/aws-samples/emr-studio-notebook-examples/blob/bb73023aa1f7b018e15de1bd90aa411f4536f143/examples/jupyter-magic-commands.ipynb#L125

Using Test PyPI is extremely questionable, and using _real PyPI_ behind it as an extra index is even more so. I recommend removing both of those options, sticking with a normal `pip install emr-notebooks-magics`.

> [!NOTE]
> `emr-notebooks-magics` also appears to no longer be maintained.

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I'm not sure if this repo is being maintained either -- 3 years since last commit -- but it is being presented as part of a user's first interaction with EMR Serverless, so I recommend at least taking this opportunity to avoid misleading users about proper `pip` usage.