docs(changelog): add link reference (#635)

Author: naveensingh

CHANGELOG.md

@@ -9,7 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
-- Holidays for Vietnam
+- Holidays for Vietnam ([#613])
 
 ### Changed
 
@@ -128,4 +128,5 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 [#262]: https://github.com/FossifyOrg/Calendar/issues/262
 [#484]: https://github.com/FossifyOrg/Calendar/issues/484
 [#551]: https://github.com/FossifyOrg/Calendar/issues/551
-[#567]: https://github.com/FossifyOrg/Calendar/issues/567
+[#567]: https://github.com/FossifyOrg/Calendar/issues/567
+[#613]: https://github.com/FossifyOrg/Calendar/issues/613