Fix date in changelog

Author: Ioannis Giagkiozis

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](https://semver.org/spec/v2.0.0.html).
 
-## [0.7.0] - 2021-xx-xx
+## [0.7.0] - 2022-01-01
 ### Added
 - `ScatterPolar` contributed by @samlich.
 -  Add a method to render the plot to anything implementing Write, contributed by @foresterre.