Doc update Quickstart Example

[eroell]

What does this PR do?

Fixing #496

This is fixing the mentioned issue with as minimal changes as possible, keeping the minimality of this example. But I've seen that in the notebook the imputaton example with saits has already been extended to newer releases from pypots and tsdb.

If this is helpful, feel free to modify the PR such that it is useful for your quickstart example. :) I'm aware that maybe load_specific_dataset could be changed to use BenchPOTS or so, but as this would make the example less minimal for now I refrained from doing so yet...

Best,

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