update kn func snippet to suggest use of registry flag if not build

[dprotaso]

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[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso

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[dprotaso]

/cherry-pick release-1.20

[knative-prow-robot]

@dprotaso: once the present PR merges, I will cherry-pick it on top of release-1.20 in a new PR and assign it to you.

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In response to this:

/cherry-pick release-1.20

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[Leo6Leo]

/lgtm

[knative-prow-robot]

@dprotaso: new pull request created: #6513

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In response to this:

/cherry-pick release-1.20

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.