Improve legibility of RunOnce::run_unsafe param (#2181)

During PR #2046 @cart suggested that the `(): ()` notation is less legible than `_input: ()`. The first notation still managed to slip in though. This PR applies the second writing.

crates/bevy_ecs/src/schedule/run_criteria.rs

@@ -406,7 +406,7 @@ impl System for RunOnce {
         true
     }
 
-    unsafe fn run_unsafe(&mut self, (): (), _world: &World) -> ShouldRun {
+    unsafe fn run_unsafe(&mut self, _input: (), _world: &World) -> ShouldRun {
         if self.ran {
             ShouldRun::No
         } else {