docs: Fix arrays in data set

[giulioungaretti]

See QCoDeS/Qcodes_loop#20 .

Changes proposed in this pull request:

• Fix the docs

@alexcjohnson say the word, as that's your code.

It seems the attribute and the arguments where mixed up!

[qcodes-bot]

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[alexcjohnson]

💃
Right, the constructor arg and the corresponding (resulting) attribute do not match. Dunno if this is something we want to change in the future so they DO match, that would be nice but:

• as a constructor arg, a dict would be redundant because you need to map {array_id:array} when array_id is already specified inside each array
• but as an attribute, we need to be able to efficiently look up arrays by their ids
  So I'd vote to just make it clear that the arg and the attribute are different.

[giulioungaretti]

I think it's fine like this with the docs fixed.