[stable/nginx-ingress] Document full key for external-dns example

[AndiDog]

What this PR does / why we need it:

Completes the external-dns example using the full nested key, so it could be copy-pasted without wondering where the annotations key must go.

Special notes for your reviewer:

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• title of the PR contains starts with chart name e.g. [stable/chart]

@jackzampolin @mgoodness

[k8s-ci-robot]

Hi @AndiDog. Thanks for your PR.

I'm waiting for a helm 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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[ChiefAlexander]

/assign
/ok-to-test

Thanks for updating the docs!

[AndiDog]

/retest

[ChiefAlexander]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AndiDog, ChiefAlexander

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