Update join Slack link

[Syulin7]

What this PR does / why we need it:
Update it to https://www.kubeflow.org/docs/about/community/#kubeflow-slack which will always have a working link. (rather than using the invited link directly).

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):
fix: kubeflow/community#602
like kubeflow/community#604

Checklist:

• Docs included if any changes are user facing

[Syulin7]

@tenzen-y @johnugeorge PTAL, Thanks.

[tenzen-y]

Looks good.
Thanks for the updates!

/lgtm

/assign @gaocegege @johnugeorge @terrytangyuan

[johnugeorge]

Sorry for missing this

/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnugeorge, Syulin7

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• OWNERS [johnugeorge]

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[coveralls]

Pull Request Test Coverage Report for Build 4080282064

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 14 unchanged lines in 2 files lost coverage.
• Overall coverage remained the same at 39.303%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	5	76.97%
pkg/controller.v1/pytorch/pytorchjob_controller.go	9	57.87%

Totals
Change from base Build 4066710325:	0.0%
Covered Lines:	2719
Relevant Lines:	6918

---

💛 - Coveralls