Fix ociReference formatting in collection-index.yml

[AntonZheng]

What type of PR is this?

• [ ] Add a new dev container collection
• [x] Update to an existing dev container collection
• [ ] Documentation/spec update
• [ ] Other containers.dev site update (UX, layout, etc)

Description

following previous pr: https://github.com/devcontainers/devcontainers.github.io/pull/607

While the collection was showing up in https://containers.dev/collections I don't see the template showing up https://containers.dev/templates

Think this could be because we reference the template itself instead of the collection as the oci reference. PR is to update to point to the collection itself.