Add fix description

Signed-off-by: naomichi-y <n.yamakita@gmail.com>

CHANGELOG.md

@@ -57,6 +57,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Client with Java 8 runtime and Apache HttpClient 5 Transport fails with java.lang.NoSuchMethodError: java.nio.ByteBuffer.flip()Ljava/nio/ByteBuffer ([#13100](https://github.com/opensearch-project/opensearch-java/pull/13100))
 - Fix implement mark() and markSupported() in class FilterStreamInput ([#13098](https://github.com/opensearch-project/OpenSearch/pull/13098))
 - Fix snapshot _status API to return correct status for partial snapshots ([#12812](https://github.com/opensearch-project/OpenSearch/pull/12812))
+- Fixed array field name omission in flat_object function for nested JSON ([#13620](https://github.com/opensearch-project/OpenSearch/pull/13620))
 
 ### Security