Update changelog

Author: olgabrani

CHANGELOG.md

@@ -4,6 +4,12 @@ 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).
 
+## [v2.3.1](https://github.com/rciam/simplesamlphp-module-themevanilla/compare/v2.3.0...v2.3.1) - 2019-09-12
+
+### Fixed
+
+- Fix bug in IdPs buttons style caused by unclosed html element.
+
 ## [v2.3.0](https://github.com/rciam/simplesamlphp-module-themevanilla/compare/v2.2.1...v2.3.0) - 2019-08-01
 
 ### Added