Fix typo in Future.then documentation

lib/core/future.dart

Index: lib/core/future.dart
diff --git a/lib/core/future.dart b/lib/core/future.dart
index 255b67802b03f80f514545a1841773aa1841dfe7..148620295e1fb087f5d48e5aee7c1dbd6917cf7f 100644
--- a/lib/core/future.dart
+++ b/lib/core/future.dart
@@ -70,7 +70,7 @@ interface Future<T> default FutureImpl<T> {
   void onComplete(void complete(Future<T> future));
 
   /**
-   * If this future is complete and has a value, then [onValue] is called
+   * If this future is complete and has a value, then [onSuccess] is called
    * with the value.
    */
   void then(void onSuccess(T value));