Update creating-a-widget.md (#518)

Adds `import 'package:flutter/widgets.dart';` to the button spec generation example for the generated `ButtonSpecTween` class.

website/pages/docs/tutorials/creating-a-widget.md

@@ -94,6 +94,7 @@ A Spec, which is short for Specification, is the visual properties and attribute
 This will look something like this:
 
 ```dart
+import 'package:flutter/widgets.dart';
 import 'package:mix/mix.dart';
 import 'package:mix_annotations/mix_annotations.dart';