[BUG-REPORT] jupyter notebook error in tutorial docs

[topspinj]

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Description
Please provide a clear and concise description of the problem. This should contain all the steps needed to reproduce the problem. A minimal code example that exposes the problem is very appreciated.

Hey there.

I noticed a "df is not found" error in the docs: https://vaex.io/docs/tutorial.html#DataFrame
I suspect it's likely a jupyter notebook issue. Looks like that cell was run separately from the rest of the code.

Software information

• Vaex version (import vaex; vaex.__version__):
• Vaex was installed via: pip / conda-forge / from source
• OS:

Additional information
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