add missing link to Changelog

Author: barryorourke

CHANGELOG.md

@@ -3,7 +3,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 This CHANGELOG follows the format listed [here](https://github.com/sensu-plugins/community/blob/master/HOW_WE_CHANGELOG.md)
 
-## Unreleased
+## [Unreleased]
 ### Added
 - Add support for python 3.5, which is the default version in Debian 9. (@barryorourke)