removing an inaccurate underscore

src/main.rs

@@ -103,8 +103,8 @@ fn event(app: &App, model: &mut Model, event: WindowEvent) {
 }
 
 // Update the state of your application here. By default, this gets called right before `view`.
-fn update(_app: &App, _model: &mut Model, _update: Update) {
-    _model.engine.step();
+fn update(_app: &App, model: &mut Model, _update: Update) {
+    model.engine.step();
 }
 
 // Draw the state of your `Model` into the given `Frame` here.