Rename example system per review feedback

Author: LegNeato

examples/app/headless_defaults.rs

@@ -15,15 +15,15 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
-        .add_system(do_something)
+        .add_system(exit_unconditionally)
         .run();
 }
 
 // Normally Bevy exits when all windows are closed.
 //
 // When running in headless mode there are no windows so
 // you must manually send an [`bevy::app::AppExit`] event.
-fn do_something(mut app_exit_events: EventWriter<AppExit>) {
+fn exit_unconditionally(mut app_exit_events: EventWriter<AppExit>) {
     info!("Successfully ran! Exiting...");
     app_exit_events.send(AppExit);
 }