fix docs on ghcr repo linking via docker labels

[hardbyte]

The currently documented method for linking GitHub repositories to GHCR packages doesn't work as the suggested syntax isn't correct.

What article on docs.github.com is affected?

• https://docs.github.com/en/free-pro-team@latest/packages/guides/connecting-a-repository-to-a-container-image
• Also the help text at https://github.com/orgs/<ORG NAME>/packages/container/package/<PACKAGE Name> (pictured below)

What part(s) of the article would you like to see updated?

Docker labels are specified in Dockerfiles with an equals sign as per official documentation (already linked in the docs).

The currently documented method for linking GitHub repositories to GHCR packages isn't valid.

Additional information

[welcome]

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