Correct mistake of Migration from Vue 1.x (#1686)

Author: CyanSalt

src/v2/guide/migration.md

@@ -364,7 +364,7 @@ computed: {
 Or with component methods:
 
 ``` js
-template: '<p>message: {{ getTimeMessage }}</p>',
+template: '<p>message: {{ getTimeMessage() }}</p>',
 methods: {
   getTimeMessage: function () {
     return Date.now() + this.message