Update README.md to reflect the correct Go Link

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Description

The Go Link for Federated Plugin Implementation Document was incorrect.
Pushing the change with corrected link.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
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• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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[stuartmorgan-g]

Thanks for the submission! We apologize for the long delay in triaging this PR. We’re in the process of overhauling our PR triage system to respond much more quickly, as well as working through the backlog.

Unfortunately we ended up with a duplicate PR in #4143, which has now landed, so closing this as obsolete.