Update URL for HyP3 sample notebook

[hjkristenson]

Description of proposed changes

The link to the smallbaselineApp_hyp3.ipynb tutorial was broken in https://github.com/insarlab/MintPy/blob/main/docs/dir_structure.md. It looks like that notebook was moved into a workflows subdirectory. This PR updates the URL to the current notebook location.

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[yunjunz]

LGTM.

[yunjunz]

Thank you @hjkristenson for the fix. I will merge it right away.

When you get a chance, could you take a look at #1004 also?

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