[doc] [dask] Add example on early stopping with Dask#6501

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trivialfis merged 2 commits intodmlc:masterfrom jameslamb:docs/dask-early-stopping

Dec 15, 2020

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Commits on Dec 14, 2020

[doc] [dask] Add example on early stopping with Dask
jameslamb
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Commits on Dec 15, 2020

Typo, model slice.
trivialfis
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