fix new clippy error (#656)

crates/bevy_app/src/event.rs

@@ -229,7 +229,7 @@ impl<T: bevy_ecs::Resource> Events<T> {
     }
 
     /// Creates a draining iterator that removes all events.
-    pub fn drain<'a>(&'a mut self) -> impl Iterator<Item = T> + 'a {
+    pub fn drain(&mut self) -> impl Iterator<Item = T> + '_ {
         let map = |i: EventInstance<T>| i.event;
         match self.state {
             State::A => self