fix observer docs (#15415)

# Objective

- #15331

## Solution

-Just changed it to Trigger since the function signature shows it's just
a wrapper trait

## Testing

Will let tests pass

---------

Co-authored-by: Fernan Lukban <[email protected]>
Co-authored-by: Alice Cecile <[email protected]>
Co-authored-by: Antony <[email protected]>

crates/bevy_ecs/src/system/observer_system.rs

@@ -5,9 +5,7 @@ use crate::{
 
 use super::IntoSystem;
 
-/// Implemented for systems that have an [`Observer`] as the first argument.
-///
-/// [`Observer`]: crate::observer::Observer
+/// Implemented for [`System`]s that have a [`Trigger`] as the first argument.
 pub trait ObserverSystem<E: 'static, B: Bundle, Out = ()>:
     System<In = Trigger<'static, E, B>, Out = Out> + Send + 'static
 {