Updating Changelog file for bug fix #2556

Signed-off-by: Arun Ganesh <oaganesh@amazon.com>

CHANGELOG.md

@@ -14,6 +14,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Enhancements
 * Introduce node level circuit breakers for k-NN [#2509](https://github.com/opensearch-project/k-NN/pull/2509)
 ### Bug Fixes
+* Fixing bug to prevent NullPointerException while doing PUT mappings [#2556](https://github.com/opensearch-project/k-NN/issues/2556)
 ### Infrastructure
 * Removed JDK 11 and 17 version from CI runs [#1921](https://github.com/opensearch-project/k-NN/pull/1921)
 * Upgrade min JDK compatibility to JDK 21 [#2422](https://github.com/opensearch-project/k-NN/pull/2422)