Update CHANGELOG.md

Author: Kludex

CHANGELOG.md

@@ -11,7 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
-- `opentelemetry-instrumentation-httpx` Instrument transport instead of HTTPX client
+- `opentelemetry-instrumentation-httpx` Add the `is_instrumented` flag to the transport
   ([#3106](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3106))
 
 ## Version 1.29.0/0.50b0 (2024-12-11)