fix module name of k8s-manifest

[loomsen]

When using kubernetes-manifest as per the docs, we get an error upon generate-terraform

ERROR: kubernetes-manifest is not a valid module type
If you need more help please reach out to the contributors in our slack channel at: https://slack.opta.dev/
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User on the latest version.

Checking the source:
https://github.com/run-x/opta/tree/cfd4ddd81a47320df33ca15fc0f4814c79836f38/modules/k8s_manifest

The module is called k8s-manifest, not kubernetes-manifest

With k8s-manifest the module is found (although there is currently another bug in the module itself).

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