Fix broken kafka source link

[qiujian16]

Fixes #1780

Proposed Changes

• kafka source link is broken so change to another location

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[knative-prow-robot]

Hi @qiujian16. Thanks for your PR.

I'm waiting for a knative 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.

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[googlebot]

CLAs look good, thanks!

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[gyliu513]

/ok-to-test
/lgtm

[samodell]

/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiujian16, samodell

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