Remove dead link to kubeflow community slack

[anishasthana]

The contributing.md file links to a now archived channel in the old kubeflow slack. This commit removes that link unitl we have created a new #community channel in the CNCF slack

[google-oss-prow]

Hi @anishasthana. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[terrytangyuan]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: terrytangyuan

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