Update batch-linux-nodes.md

[pstakashi]

Changed to latest image reference.

I could not find any image in existing image(Publisher: Canonical, Offer: Ubuntuserver, SKU: 20.04-LTS), failed to add pool. After changed to the latest image, successfully added.

[pstakashi]

@microsoft-github-policy-service agree

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[Padmalathas]

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