Update README.md example to use Invocation

PiperOrigin-RevId: 291036628
diff --git a/README.md b/README.md
index 5b70df8..09b6878 100644
--- a/README.md
+++ b/README.md
@@ -65,7 +65,7 @@
 
 // We can calculate a response at call time.
 var responses = ["Purr", "Meow"];
-when(cat.sound()).thenAnswer(() => responses.removeAt(0));
+when(cat.sound()).thenAnswer((_) => responses.removeAt(0));
 expect(cat.sound(), "Purr");
 expect(cat.sound(), "Meow");
 ```