Fix v0.3.4 markdown syntax in CHANGELOG.md

Author: griff-rees

CHANGELOG.md

@@ -5,7 +5,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 Please maintain both this file AND the `README.md` file.
 
-## [0.3.4]
+## v0.3.4
 
 ### Added
  * Added `PyPI` version and `MIT` license badges to `README.md`