Reference addition of ActionMailer in 0.53.0 changelog

This was added in #1280 and it's definitely relevant enough to include
in the changelog :)

CHANGELOG.md

@@ -17,6 +17,7 @@ Git diff: https://github.com/DataDog/dd-trace-rb/compare/v0.52.0...v0.53.0
 - Runtime Metrics: Global VM cache statistics ([#1680][])
 - Automatically send traces to agent Unix socket if present ([#1700][])
 - CI-App: User Provided Git Metadata ([#1662][])
+- ActionMailer integration ([#1280])
 
 ### Changed
 
@@ -2730,4 +2731,4 @@ Git diff: https://github.com/DataDog/dd-trace-rb/compare/v0.3.0...v0.3.1
 [@vramaiah]: https://github.com/vramaiah
 [@walterking]: https://github.com/walterking
 [@y-yagi]: https://github.com/y-yagi
-[@zachmccormick]: https://github.com/zachmccormick
+[@zachmccormick]: https://github.com/zachmccormick