Use dict in docs Dataset construction

[max-sixty]

• Passes pre-commit run --all-files

As discussed in other issues — I find this way significantly easier to read, particularly when we're passing tuples of tuples as part of the construction. I did these manually but can write something to do it more broadly if people agree.

[shoyer]

Personally I could go either way here. I find this roughly similarly readable, on average.

[max-sixty]

OK, I'll merge this but wait for more votes before expanding any further