Update dns-stack.md reference

[emmanuel-ferdman]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Commit 4dbfd42 moved the location of dns-stack.md. This PR adjusts sources to changes.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Update `dns-stack.md` reference in `docs/ansible/vars.md`

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[k8s-ci-robot]

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[VannTen]

/ok-to-test
/label ci-short
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: emmanuel-ferdman, VannTen

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