Fix wrong version in CHANGES.md

Author: nulltoken

CHANGES.md

@@ -10,7 +10,7 @@
   - CodeBetter TeamCity: <http://teamcity.codebetter.com/project.html?projectId=LibGit2Sharp&guest=1>
   - Travis: <https://travis-ci.org/libgit2/libgit2sharp>
 
-## v0.21 - ([diff](https://github.com/libgit2/libgit2sharp/compare/v0.20...v0.20.1))
+## v0.20.1 - ([diff](https://github.com/libgit2/libgit2sharp/compare/v0.20...v0.20.1))
 
 ### Fixes