Update CHANGELOG.md

Author: cristianoc

CHANGELOG.md

@@ -26,6 +26,7 @@
 - AST cleanup: first-class expression and patterns for records with optional fields. https://github.com/rescript-lang/rescript/pull/7192
 - AST cleanup: Represent the arity of uncurried function definitions directly in the AST. https://github.com/rescript-lang/rescript/pull/7197
 - AST cleanup: Remove Pexp_function from the AST. https://github.com/rescript-lang/rescript/pull/7198
+- AST cleanup: Remove `Function$` entirely for function definitions. https://github.com/rescript-lang/rescript/pull/7200
 
 
 # 12.0.0-alpha.5