Update application-config.md

vuejs/docs@1e772b3#diff-4bcba432c725c343e09028bc45d74c2a76af05e7148214dc7da62a8aace1718b

Author: naokie

src/api/application-config.md

@@ -6,6 +6,8 @@
 const app = Vue.createApp({})
 
 app.config = {...}
+
+app.mount(...);
 ```
 
 ## errorHandler