Update aws-s3-key.yaml reference URL

[hahwul]

Updated the reference field in secrets/aws-s3-key.yaml to include the URL https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_access-keys.html.

This change addresses the missing documentation link for the S3 Access Key detection rule. The reference points to the AWS documentation for managing access keys, which are the credentials detected by this rule.

Verified the YAML syntax using yamllint.

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PR created automatically by Jules for task 495082897980728941 started by @hahwul

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