update CHANGELOG.md

Author: herin049

CHANGELOG.md

@@ -40,6 +40,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#3967](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3967))
 - `opentelemetry-instrumentation-redis`: add missing copyright header for opentelemetry-instrumentation-redis
   ([#3976](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3976))
+- `opentelemetry-instrumentation-aiohttp-client`: add ability to capture custom headers
+  ([#3988](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3988))
 
 ### Fixed