Add missing colon in States documentation (#11064)

# Objective

The documentation for the `States` trait contains an error! There is a
single colon missing from `OnExit<T:Variant>`.

## Solution

Replace `OnExit<T:Variant>` with `OnExit<T::Variant>`. (Notice the added
colon.)

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## Changelog

### Added

- Added missing colon in `States` documentation.

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Bevy community, you may now rest easy.

crates/bevy_ecs/src/schedule/state.rs

@@ -24,7 +24,7 @@ pub use bevy_ecs_macros::States;
 /// You can access the current state of type `T` with the [`State<T>`] resource,
 /// and the queued state with the [`NextState<T>`] resource.
 ///
-/// State transitions typically occur in the [`OnEnter<T::Variant>`] and [`OnExit<T:Variant>`] schedules,
+/// State transitions typically occur in the [`OnEnter<T::Variant>`] and [`OnExit<T::Variant>`] schedules,
 /// which can be run via the [`apply_state_transition::<T>`] system.
 ///
 /// # Example