Update CHANGELOG.md

Author: oleibman

CHANGELOG.md

@@ -30,6 +30,7 @@ and this project adheres to [Semantic Versioning](https://semver.org). Thia is a
 ### Fixed
 
 - Php8.5 deprecates use of null as array index. [PR #4634](https://github.com/PHPOffice/PhpSpreadsheet/pull/4634)
+- Use prefix _xlfn for BASE function. [Issue #4638](https://github.com/PHPOffice/PhpSpreadsheet/issues/4638) [PR #4641](https://github.com/PHPOffice/PhpSpreadsheet/pull/4641)
 
 ## 2025-09-03 - 5.1.0