Update CHANGELOG for 3.10.0

Author: adriandole

CHANGELOG.md

@@ -7,7 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 Changes prior to 3.9.0 are documented as [release notes on GitHub](https://github.com/mongodb/mongo-cxx-driver/releases).
 
-## 3.10.0 [Unreleased]
+## 3.10.0
 
 ### Added