docs: remove stale ML tutorial reference

[nightcityblade]

Fixes #234

Removes the reference to notebook 08 in the Dask tutorial, which no longer contains the linked machine-learning lesson. The overview now directs readers to the Dask-ML documentation and the repository's other machine-learning examples.

Tests:

• pre-commit run --files machine-learning.ipynb
• jq empty machine-learning.ipynb
• git diff --check

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[ncclementi]

Thanks for the PR, just a small suggestion

[ncclementi]

Just point to dask ml docs, fix typo

Suggested change

	"Visit the main [Dask-ML](http://ml.dask.org) documentation or explore some of the other machine-learning examples."
	"Visit the main [Dask-ML](http://ml.dask.org) documentation to explore some of the other machine-learning examples."

[nightcityblade]

Updated in 8b8117f: the sentence now points directly to the Dask-ML docs and includes the typo correction from the suggestion.

Validation: python3 -m json.tool machine-learning.ipynb, git diff --check, and pre-commit run --files machine-learning.ipynb all pass (nbstripout passed). Thanks!

[ncclementi]

Looks like CI is failing, because the action are too old. Your contribution is good, the problem is this will not get deployed.
There are currently no efforts in maintaining this repository. So the changes won't be reflected.

Apologies. I'll leave it open in case a maintainer picks up updating the actions, but this repository hasn't seen work in a while, so I doubt it happens.

[nightcityblade]

Thanks for reviewing and for the context. Understood — I'll leave the patch open as-is in case the repository's actions are revived.