Change README.md to prefer async/await to Future.value. Fixes #261

PiperOrigin-RevId: 322853868
diff --git a/README.md b/README.md
index c54bdca..a6c8a38 100644
--- a/README.md
+++ b/README.md
@@ -111,7 +111,7 @@
 
 // GOOD
 when(mock.methodThatReturnsAFuture())
-    .thenAnswer((_) => Future.value('Stub'));
+    .thenAnswer((_) async => 'Stub');
 when(mock.methodThatReturnsAStream())
     .thenAnswer((_) => Stream.fromIterable(['Stub']));