Example for `on_event` not up to date with changes made in 0.11

[Purfakt]

How can Bevy's documentation be improved?

The current (0.12.1) example in the documentation for on_event is not up to date with changes made in 0.11.

app.add_systems(
    my_system.run_if(on_event::<MyEvent>()),
);

// [...]

I expected it to be something like:

app.add_systems(
    Update, my_system.run_if(on_event::<MyEvent>()),
);

// [...]

[alice-i-cecile]

The perils of mocking the bevy_app types for our doc tests. Thanks for spotting this!