update changelog

Author: RobinMalfait

CHANGELOG.md

@@ -7,7 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
-- Nothing yet!
+### Fixed
+
+- Use `length` data type for `background-size` instead of `background-position` ([#13771](https://github.com/tailwindlabs/tailwindcss/pull/13771))
 
 ## [4.0.0-alpha.15] - 2024-05-08