Fix custom_loop example to include plugin finalization (#13215)

# Objective

The `custom_loop` example didn't replicate the `app.finish` /
`app.cleanup` calls from the default runner; I discovered this when
trying to troubleshoot why my application with a custom loop wasn't
calling its plugin finalizers, and realised that the upstream example
that I'd referenced didn't have the relevant calls.

## Solution

Added the missing calls, replicating what the default runner does:

https://github.com/bevyengine/bevy/blob/d3904200939ee6b2c0ee5f9293ad35ba8315cbfc/crates/bevy_app/src/app.rs#L895-L896

## Testing

I've confirmed that adding these two calls to my application fixed the
issue I was encountering. I haven't tested it within the example itself
as it's relatively straightforward and I didn't want to pollute the
example with a plugin using a finalizer.

examples/app/custom_loop.rs

@@ -8,6 +8,11 @@ use std::io;
 struct Input(String);
 
 fn my_runner(mut app: App) -> AppExit {
+    // Finalize plugin building, including running any necessary clean-up.
+    // This is normally completed by the default runner.
+    app.finish();
+    app.cleanup();
+
     println!("Type stuff into the console");
     for line in io::stdin().lines() {
         {