update changelog with PR number

Author: yiyuan-he

instrumentation-genai/opentelemetry-instrumentation-langchain/CHANGELOG.md

@@ -8,6 +8,6 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Unreleased
 
 - Added agent span support for GenAI LangChain instrumentation with `invoke_agent` operation and chain tracking.
-  ([#TBD](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/TBD))
+  ([#3788](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3788))
 - Added span support for genAI langchain llm invocation.
-  ([#3665](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3665))
+  ([#3665](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3665))