Update application-config.md (vuejs#704)

I added this app.mount to the code block to visual more what the before mounting your application means. 
This way it´s clear to check that the mount() is after the config. I personal used time to asking why my code is not working and got answer that the config needs to be before mount. After this I readed again the documentation and saw the first lines "You can modify its properties listed below before mounting your application"

src/api/application-config.md

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