Fix changelog

Author: david-gang

CHANGELOG.md

@@ -13,7 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- `opentelemetry-instrumentation-system-metrics`: Add `cpython.gc.collected_objects` and `cpython.gc.uncollectable_objects` metrics
+- `opentelemetry-instrumentation-system-metrics`: Add `cpython.gc.collected_objects` and `cpython.gc.uncollectable_objects` metrics ([#3666](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3666))
 
 ## Version 1.36.0/0.57b0 (2025-07-29)