Add changelog entry for base64 dependency update

Author: mohe2015

CHANGELOG.md

@@ -6,6 +6,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Github workflow for changelog verification ([#89](https://github.com/opensearch-project/opensearch-rs/pull/89))
 
 ### Changed
+- Update base64 dependency from ^0.13 to ^0.20 ([#95](https://github.com/opensearch-project/opensearch-rs/pull/95))
 
 ### Deprecated
 
@@ -16,4 +17,4 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Security
 
 
-[Unreleased]: https://github.com/opensearch-project/opensearch-rs/compare/2.0...HEAD
+[Unreleased]: https://github.com/opensearch-project/opensearch-rs/compare/2.0...HEAD