Fix error in sample code (#10339)

packages/flutter/lib/src/animation/tween.dart

@@ -100,7 +100,7 @@ class _ChainedEvaluation<T> extends Animatable<T> {
 /// _animation = new Tween<Offset>(
 ///   begin: const Offset(100.0, 50.0),
 ///   end: const Offset(200.0, 300.0),
-/// ).animation(_controller);
+/// ).animate(_controller);
 /// ```
 ///
 /// That would provide an `_animation` that, over the lifetime of the