Add PR #18 reference to changelog

Author: igorbelo

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [0.5.1](https://github.com/fishbrain/tarpon/compare/v0.5.0...v0.5.1)
 
 ### Added
-- Support updating subscriber attributes from [@MohamedBng](https://github.com/MohamedBng) and [@Spone](https://github.com/Spone)
+- Support updating subscriber attributes from [@MohamedBng](https://github.com/MohamedBng) and [@Spone](https://github.com/Spone) (PR [#18](https://github.com/fishbrain/tarpon/pull/18))
 
 ## [0.5.0](https://github.com/fishbrain/tarpon/compare/v0.4.0...v0.5.0) - 2022-12-01