fix example in README

Author: joseph-gio

crates/bevy_ecs/README.md

@@ -148,9 +148,13 @@ fn main() {
     // Create a new Schedule, which defines an execution strategy for Systems
     let mut schedule = Schedule::default();
 
+    // Define a unique public name for a new Stage.
+    #[derive(StageLabel)]
+    pub struct UpdateLabel;
+
     // Add a Stage to our schedule. Each Stage in a schedule runs all of its systems
     // before moving on to the next Stage
-    schedule.add_stage("update", SystemStage::parallel()
+    schedule.add_stage(UpdateLabel, SystemStage::parallel()
         .with_system(movement)
     );