Change function's function param to correct syntax (#4065)

Ref: https://dart.dev/guides/language/sound-problems

Co-authored-by: ven <vendethiel@hotmail.fr>

dart.html.markdown

@@ -79,7 +79,8 @@ example1() {
 
 /// Anonymous functions don't include a name
 example2() {
-  nested1(fn) {
+  //// Explicit return type.
+  nested1(void Function() fn) {
     fn();
   }
   nested1(() => print("Example2 nested 1"));