docs: update changelog with scrollbar fix

Author: TomJGooding

CHANGELOG.md

@@ -43,6 +43,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Fixed `Pilot.click` not working with `times` parameter https://github.com/Textualize/textual/pull/5398
 - Fixed select refocusing itself too late https://github.com/Textualize/textual/pull/5420
 - Fixed Log widget not refreshing on resize https://github.com/Textualize/textual/pull/5460
+- Fixed scrollbars ignoring background opacity https://github.com/Textualize/textual/issues/5458
 
 
 ## [1.0.0] - 2024-12-12