docs: update CHANGES.md for Spark Runner (#34295)

Author: twosom

CHANGES.md

@@ -91,6 +91,7 @@
 * (Java) Fixed TIME field encodings for BigQuery Storage API writes on GenericRecords ([#34059](https://github.com/apache/beam/pull/34059)).
 * (Java) Fixed a race condition in JdbcIO which could cause hangs trying to acquire a connection ([#34058](https://github.com/apache/beam/pull/34058)).
 * (Java) Fix BigQuery Storage Write compatibility with Avro 1.8 ([#34281](https://github.com/apache/beam/pull/34281)).
+* Fixed checkpoint recovery and streaming behavior in Spark Classic and Portable runner's Flatten transform by replacing queueStream with SingleEmitInputDStream ([#34080](https://github.com/apache/beam/pull/34080), [#18144](https://github.com/apache/beam/issues/18144), [#20426](https://github.com/apache/beam/issues/20426)).
 
 ## Security Fixes
 * Fixed (CVE-YYYY-NNNN)[https://www.cve.org/CVERecord?id=CVE-YYYY-NNNN] (Java/Python/Go) ([#X](https://github.com/apache/beam/issues/X)).