Fix broken link in ch1 to fast.ai datasets

[knjk04]

The link in chapter 1 to the datasets provided by fast.ai was broken. This pull request fixes the broken link.

Previously: https://course.fast.ai/datasets

Now: https://docs.fast.ai/data.external.html#datasets

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[knjk04]

Hi @jph00, would you mind taking a look?

[jph00]

Thanks for fixing it!