Correct v2.3.1 release date in CHANGELOG.md

I made most of the `CHANGELOG.md` changes 2 August, but did not push the
version to npm until 7 August. Updating the changelog just in case anyone
updates and looks back on the changelog for clues of when things happen.

CHANGELOG.md

@@ -3,7 +3,7 @@
 All notable changes to this project will be documented in this file.
 This project adheres to [Semantic Versioning](http://semver.org/).
 
-## [2.3.1] / 2 August 2018
+## [2.3.1] / 7 August 2018
 
 ### Minor