Added pull request link in changelog

Author: Garner Jervis Tan

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `opentelemetry-instrumentation-aiopg`, `opentelemetry-instrumentation-dbapi` Remove unused `database_type` parameter from `trace_integration` function.
   ([#301](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/301))
 - `opentelemetry-instrumentation-asgi` Return header values using case insensitive keys
+  ([#308](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/308))
 
 ## [0.17b0](https://github.com/open-telemetry/opentelemetry-python-contrib/releases/tag/v0.17b0) - 2021-01-20