message: subscriber: fix broken link to `format::Writer`
diff: @@ -106,7 +106,7 @@ where
  /// ```
  /// [`FormatEvent`]: format::FormatEvent
  /// [`Event`]: tracing::Event
  - /// [`Writer`]: crate::format::Writer
  + /// [`Writer`]: format::Writer
  pub fn event_format<E2>(self, e: E2) -> Subscriber<C, N, E2, W>
  where
  E2: FormatEvent<C, N> + 'static,
  