Add PR link to changelog entry

Author: iitenkida7

CHANGELOG.md

@@ -6,7 +6,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
-- [Patch] Cast `Content-Length` header value to `string` in `Http::request()` to avoid a `guzzlehttp/psr7` 2.11 deprecation (`Passing int to MessageInterface::withHeader() is deprecated`) and stay compatible with `guzzlehttp/psr7` 3.0
+- [#465](https://github.com/Shopify/shopify-api-php/pull/465) [Patch] Cast `Content-Length` header value to `string` in `Http::request()` to avoid a `guzzlehttp/psr7` 2.11 deprecation (`Passing int to MessageInterface::withHeader() is deprecated`) and stay compatible with `guzzlehttp/psr7` 3.0
 
 ## v6.1.1 - 2026-03-02
 - [#456](https://github.com/Shopify/shopify-api-php/pull/456) [Patch] Update firebase/php-jwt to ^7.0 to address security vulnerability (GHSA-2x45-7fc3-mxwq)