Update CHANGELOG.md

Author: oleibman

CHANGELOG.md

@@ -27,6 +27,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 ### Fixed
 
 - Ods Reader Sheet Names with Period. [Issue #4311](https://github.com/PHPOffice/PhpSpreadsheet/issues/4311) [PR #4313](https://github.com/PHPOffice/PhpSpreadsheet/pull/4313)
+- Mpdf and Tcpdf Hidden Columns and Merged Cells. [Issue #4319](https://github.com/PHPOffice/PhpSpreadsheet/issues/4319) [PR #4320](https://github.com/PHPOffice/PhpSpreadsheet/pull/4320)
 
 ## 2025-01-11 - 3.8.0