Update pul request URL in CHANGELOG.md

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>

Author: reta

CHANGELOG.md

@@ -8,7 +8,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Added release notes for 1.3.5 ([#4343](https://github.com/opensearch-project/OpenSearch/pull/4343))
 - Added release notes for 2.2.1 ([#4344](https://github.com/opensearch-project/OpenSearch/pull/4344))
 - Label configuration for dependabot PRs ([#4348](https://github.com/opensearch-project/OpenSearch/pull/4348))
-- Support for HTTP/2 (server-side) (https://github.com/opensearch-project/OpenSearch/issues/3651)
+- Support for HTTP/2 (server-side) ([#3847](https://github.com/opensearch-project/OpenSearch/pull/3847))
 
 ### Changed
 - Dependency updates (httpcore, mockito, slf4j, httpasyncclient, commons-codec) ([#4308](https://github.com/opensearch-project/OpenSearch/pull/4308))