renderer: add route to second screen example

src/renderer/screens/index.ts

@@ -1,4 +1,5 @@
 import ErrorScreen from '@/renderer/screens/ErrorScreen.vue'
 import MainScreen from '@/renderer/screens/MainScreen.vue'
+import SecondScreen from '@/renderer/screens/SecondScreen.vue'
 
-export { ErrorScreen, MainScreen }
+export { ErrorScreen, MainScreen, SecondScreen }