[docs] fix broken SynapseML link

[jameslamb]

SynapseML (formerly MMLSpark) maintainers recently did another docs re-organization (microsoft/SynapseML#1221), and as a result a link in LightGBM's documentation is currently broken.

From the most recent check-links job.

URL https://github.com/microsoft/SynapseML/blob/master/docs/lightgbm.md' Name the SynapseML documentation'
Parent URL file:///home/runner/work/LightGBM/LightGBM/docs/_build/html/Parallel-Learning-Guide.html, line 180, col 167
Real URL https://github.com/microsoft/SynapseML/blob/master/docs/lightgbm.md
Check time 1.243 seconds
Result Error: 404 Not Found

reference: https://github.com/microsoft/LightGBM/runs/3987915929?check_suite_focus=true

This has happened several times over the last year:

• [docs] replace broken mmlspark notebook link in docs #4303
• [docs] update link to LightGBM example in MMLSpark repo #4401
• [docs] update links to SynapseML (former MMLSpark) #4564

To fix the broken link and avoid disruptions from future re-organizations in SynapseML, this PR proposes replacing the LightGBM-specific link that is now broken to just link to the root of the SynapseML repo instead.

cc @imatiach-msft

[imatiach-msft]

@jameslamb sounds good to me, sorry about breaking you so often! Also FYI @serena-ruan

[jameslamb]

sorry about breaking you so often

no problem, just wanted you to be aware!

[StrikerRUS]

@jameslamb Maybe we can just remove ... and the SynapseML documentation ... part from the original sentence because this new very general link to the SynapseML repo's root doesn't look very useful for readers?

[jameslamb]

Maybe we can just remove ... and the SynapseML documentation ... part from the original sentence

Yeah I agree! Good point. I just pushed a commit removing that.

[StrikerRUS]

Thanks!

[github-actions]

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