message: Fix Event::all_fields doctest
diff: @@ -255,11 +255,13 @@ impl<'event, 'meta: 'event> Event<'event, 'meta> {
  /// # #[macro_use]
  /// # extern crate tokio_trace;
  /// # use tokio_trace::Level;
  + /// # fn main() {
  /// span!("parent 1", foo = 1, bar = 1).enter(|| {
  /// span!("parent 2", foo = 2, bar = 1).enter(|| {
  /// event!(Level::Info, { bar = 2 }, "my event");
  /// })
  - /// })
  + /// });
  + /// # }
  /// ```
  /// If a `Subscriber` were to call `all_fields` on this event, it will
  /// receive an iterator with the values `("foo", 2)` and `("bar", 2)`.
  