message: subscriber: fix typo in docs
  ## Motivation
  It's a typo in docs.
  ## Solution
  Replacing  `whicb` with `which`.
diff: @@ -28,7 +28,7 @@ cfg_feature!("registry", {
  /// instead, it collects and stores span data that is exposed to any `Subscriber`s
  /// wrapping it through implementations of the [`LookupSpan`] trait.
  /// The `Registry` is responsible for storing span metadata, recording
  - /// relationships between spans, and tracking which spans are active and whicb
  + /// relationships between spans, and tracking which spans are active and which
  /// are closed. In addition, it provides a mechanism for `Subscriber`s to store
  /// user-defined per-span data, called [extensions], in the registry. This
  /// allows `Subscriber`-specific data to benefit from the `Registry`'s
  