Use a better example of overriding fields in the changelog.

CHANGELOG.md

 ## 1.24.0
 
 ### Language
 
 #### Strong Mode
 
 ### Core library changes
 
 * `dart:io`
   * Added `Platform.localeName`, needed for accessing the locale on platforms
@@ skipping 78 matching lines @@
   ```
 
 * Strong mode supports overriding fields, `@virtual` is no longer required
     (SDK issue [28120](https://github.com/dart-lang/sdk/issues/28120)).
 
     ```dart
     class C {
       int x = 42;
     }
     class D extends C {
-      int x = 123;
-      get y => super.x;
+      get x {
+        print("x got called");
+        return super.x;
+      }
     }
     main() {
       print(new D().x);
-      print(new D().y);
     }
     ```
 
 * Strong mode down cast composite warnings are no longer issued by default.
   (SDK issue [28588](https://github.com/dart-lang/sdk/issues/28588)).
 
 ```dart
 void test() {
   List untyped = [];
   List<int> typed = untyped; // No down cast composite warning
@@ skipping 1756 matching lines @@
     they will keep the Dart process alive until they time out. This fixes the
     handling of persistent connections. Previously, the client would shut down
     immediately after a request.
 
   * **Breaking change:** `HttpServer` no longer compresses all traffic by
     default. The new `autoCompress` property can be set to `true` to re-enable
     compression.
 
 * `dart:isolate`: `Isolate.spawnUri` added the optional `packageRoot` argument,
   which controls how it resolves `package:` URIs.