Fix markdown in changelog.

Author: jarnovanleeuwen

CHANGELOG.md

@@ -5,10 +5,10 @@ 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]
-## Added
+### Added
 - Added a test suite.
 
-## Changed
+### Changed
 - PHPTikkie now requires PHP 7.1+
 
 ## [0.1.2] - 2018-06-19