update changelog for 6.4.1 (#4399)

Signed-off-by: Justin Kolberg <amd.prophet@gmail.com>

Author: amdprophet

CHANGELOG.md

@@ -6,7 +6,7 @@ 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).
 
-## Unreleased
+## [6.4.1] - 2021-08-24
 
 ### Fixed
 - Sensu Go OSS can now be built on `darwin/arm64`.