[ServiceBus] Update changelog for external contribution (Azure#22304)

* add changelog to appreicate external contributor

* typo

Author: yunhaoling

sdk/servicebus/azure-servicebus/CHANGELOG.md

@@ -8,6 +8,8 @@
 
 ### Bugs Fixed
 
+- Fixed bug that when setting `ServiceBusMessage.time_to_live` with value being `datetime.timedelta`, `total_seconds` should be respected (PR #21869, thanks @jyggen).
+
 ### Other Changes
 
 ## 7.4.0 (2021-11-09)