fix import in code sample

Author: yyx990803

src/guide/components/async.md

@@ -5,7 +5,7 @@
 In large applications, we may need to divide the app into smaller chunks and only load a component from the server when it's needed. To make that possible, Vue has a [`defineAsyncComponent`](/api/general.html#defineasynccomponent) method:
 
 ```js
-const { defineAsyncComponent } = Vue
+import { defineAsyncComponent } from 'vue'
 
 const AsyncComp = defineAsyncComponent(() => {
   return new Promise((resolve, reject) => {