[docs] replace broken mmlspark notebook link in docs

[jameslamb]

The most recent link checks job failed.

URL        `https://github.com/Azure/mmlspark/blob/master/notebooks/samples/LightGBM%20-%20Quantile%20Regression%20for%20Drug%20Discovery.ipynb'
Name       `this MMLSpark example'
Parent URL file:///home/runner/work/LightGBM/LightGBM/docs/_build/html/Parallel-Learning-Guide.html, line 273, col 8
Real URL   https://github.com/Azure/mmlspark/blob/master/notebooks/samples/LightGBM%20-%20Quantile%20Regression%20for%20Drug%20Discovery.ipynb
Check time 0.464 seconds
Result     Error: 404 Not Found

reference: https://github.com/microsoft/LightGBM/runs/2618067011?check_suite_focus=true

Seems that mmlspark removed their LightGBM Quantile Regression example yesterday. See microsoft/SynapseML#1043.

This PR proposes updating the documentation to point to a different LightGBM mmlspark notebook.

[StrikerRUS]

Thank you!

[StrikerRUS]

link check job is green now: https://github.com/microsoft/LightGBM/runs/2629526800?check_suite_focus=true.

[github-actions]

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