Update README

[TheHackerDev]

Fixes link to Knative build authentication docs repo.

[knative-prow-robot]

Hi @aaronhnatiw. Thanks for your PR.

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[knative-prow-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: aaronhnatiw, zouyee
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We suggest the following additional approver: mattmoor

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[zouyee]

/ok-to-test