Fix a getter name in a comment from an extension types usage example (#…

…5564)

This PR just fixes a typo in the NumberE usage example : a comment
refers to a 'i' getter when it should be the 'next' getter

src/content/language/extension-types.md

@@ -380,7 +380,7 @@ void testE() {
   
   List<NumberE> numbers = [
     NumberE(1), 
-    num1.next, // OK: 'i' getter returns type 'NumberE'.
+    num1.next, // OK: 'next' getter returns type 'NumberE'.
     1, // Error: Can't assign 'int' element to list type 'NumberE'.
   ];
 }
@@ -458,4 +458,4 @@ is their lightweight implementation, which can greatly improve performance in so
 [instance variables]: /language/classes#instance-variables
 [`external`]: /language/functions#external
 [abstract members]: /language/methods#abstract-methods
-[`is` or `as` check]: /language/operators#type-test-operators
+[`is` or `as` check]: /language/operators#type-test-operators