Update banner.html (#1708)

Author: mit-mit

src/_includes/banner.html

@@ -3,6 +3,6 @@
   <p class="banner__text">
     {% comment %} TEXT GOES HERE {% endcomment %}
     Dart 2.4.0 is live! For more information, see the
-    <a class="external" href="https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#240---2019-06-24">change log</a>.
+    <a class="external" href="https://github.com/dart-lang/sdk/blob/master/CHANGELOG.md#240---2019-06-27">change log</a>.
   </p>
 </div>