Update guides/release/testing/unit-testing-basics.md

Co-Authored-By: muziejus <[email protected]>

Author: rwjblue

guides/release/testing/unit-testing-basics.md

@@ -16,6 +16,7 @@ based on a `foo` property.
 ```javascript {data-filename=app/services/some-thing.js}
 import Service from '@ember/service';
 import { computed } from '@ember/object';
+import { tracked } from '@glimmer/tracking';
 
 export default class SomeThingService extends Service {
   @tracked foo = 'bar';