Update CHANGELOG.md

Author: keithduncan

CHANGELOG.md

@@ -4,7 +4,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
-## [v5.6.1](https://github.com/buildkite/elastic-ci-stack-for-aws/compare/v5.6.0...v5.6.1) (TBA)
+## [v5.6.1](https://github.com/buildkite/elastic-ci-stack-for-aws/compare/v5.6.0...v5.6.1) (2021-09-02)
 
 ## Fixed