Update CHANGELOG.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: Joao-Dionisio

CHANGELOG.md

@@ -10,7 +10,7 @@
 ### Fixed
 - Raised an error when an expression is used when a variable is required
 ### Changed
-- MatrixExpr.sum() could return MatrixExpr
+- MatrixExpr.sum() now supports axis arguments and can return either a scalar or MatrixExpr depending on the result dimensions
 ### Removed
 
 ## 5.5.0 - 2025.05.06