message: adapted the guides
diff: @@ -228,7 +228,7 @@ We'll use [`timer::delta`](https://docs.rs/ggez/0.7.0/ggez/timer/fn.delta.html)
  ```rust,skt-update,no_run
  fn update(&mut self, ctx: &mut Context) -> GameResult {
  - self.dt = timer::delta(ctx);
  + self.dt = ctx.time.delta();
  Ok(())
  }
  ```
  