Fix typo (#1315)

When executing command `npm run build:server` got an error: "ERROR  Build failed with errors."

Author: dews

src/guide/ssr/structure.md

@@ -147,7 +147,7 @@ import { createSSRApp } from 'vue'
 import App from './App.vue'
 
 export default function () {
-  const app = createSSRApp(Vue)
+  const app = createSSRApp(App)
 
   return {
     app