[cookbook]Change "data" to a function (#2221)

* Change "data" to a function

* Update src/v2/cookbook/unit-testing-vue-components.md

Co-Authored-By: Phan An <me@phanan.net>

Author: re4388

src/v2/cookbook/unit-testing-vue-components.md

@@ -188,7 +188,11 @@ import Foo from './Foo'
 
 const factory = (values = {}) => {
   return shallowMount(Foo, {
-    data: { ...values  }
+    data () {
+      return {
+        ...values
+      }
+    }
   })
 }