make sure all PreviousState methods are private

Signed-off-by: lee-orr <lee-orr@users.noreply.github.com>

crates/bevy_ecs/src/schedule/state.rs

@@ -403,7 +403,7 @@ struct PreviousState<S: States>(S);
 
 impl<S: States> PreviousState<S> {
  /// Get the current state.
- pub fn get(&self) -> &S {
+ fn get(&self) -> &S {
  &self.0
  }
 }