Fix moved rke2 doc link

[richard-cox]

Summary

Fixes #12933

Occurred changes and/or fixed issues

• https://docs.rke2.io/install/containerd_registry_configuration --> https://docs.rke2.io/install/private_registry
• See https://github.com/rancher/rke2-docs/pull/296/files
  • file / link was renamed to private_registry

Areas or cases that should be tested

• Create / Edit rke2 cluster --> registries --> advanced (see screenshot) --> docs link

Screenshot/Video

Checklist

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  • see check-i18n-links job
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[aalves08]

LGTM