add #556 to CHANGELOG

Author: StijnCaerts

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Fixed
 
 - Fix unawaited coroutine in `stac_fastapi.core.core`. [#551](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/551)
+- Parse `ES_TIMEOUT` environment variable as an integer. [#556](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/556)
 
 ### Removed