Outdated readme link for build secrets help

[sj26]

While setting up a new pipeline I saw:

The repository "git@github.com:..." appears to use SSH for transport, but the elastic-ci-stack-s3-secrets-hooks plugin did not find any SSH keys in the "packagecloud-buildkite-agents-development" S3 bucket.
See https://github.com/buildkite/elastic-ci-stack-for-aws#build-secrets for more information.
Evaluating 0 bytes of env

but https://github.com/buildkite/elastic-ci-stack-for-aws#build-secrets doesn't seem to go anywhere in particular on the page.