docs: keep app.vue file name consistent (#18517)

Use `app.vue` instead of `App.vue`.

docs/1.getting-started/3.views.md

@@ -32,7 +32,7 @@ Most components are reusable pieces of the user interface, like buttons and menu
 
 ::code-group
 
-```vue [App.vue]
+```vue [app.vue]
 <template>
   <div>
     <h1>Welcome to the homepage</h1>