Fix diff url in CHANGELOG.md

Author: Art4

CHANGELOG.md

@@ -5,7 +5,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [Unreleased](https://github.com/kbsali/php-redmine-api/compare/v2.1.0...v2.x)
+## [Unreleased](https://github.com/kbsali/php-redmine-api/compare/v2.1.1...v2.x)
 
 ## [v2.1.1](https://github.com/kbsali/php-redmine-api/compare/v2.1.0...v2.1.1) - 2022-01-15