message: subscriber: fix layer test error message
  ## Motivation
  Test error message was incorrect.
  ## Solution
  This PR makes a 1-character change to a test error message.
diff: @@ -1186,7 +1186,7 @@ pub(crate) mod tests {
  assert_eq!(&layer.0, "layer_1");
  let layer = Subscriber::downcast_ref::<StringLayer2>(&s).expect("layer 2 should downcast");
  assert_eq!(&layer.0, "layer_2");
  - let layer = Subscriber::downcast_ref::<StringLayer3>(&s).expect("layer 13 should downcast");
  + let layer = Subscriber::downcast_ref::<StringLayer3>(&s).expect("layer 3 should downcast");
  assert_eq!(&layer.0, "layer_3");
  }
  }
  