message: docs: minor fix to Subscriber::exit doc comment
  This fixes the `Subscriber::exit` doc comment where some copy-pasta from `Subscriber::enter` was evident.
diff: @@ -286,7 +286,7 @@ pub trait Subscriber: 'static {
  /// Records that a span has been exited.
  ///
  - /// When entering a span, this method is called to notify the subscriber
  + /// When exiting a span, this method is called to notify the subscriber
  /// that the span has been exited. The subscriber is provided with the
  /// [span ID] of the exited span, and should update any internal state
  /// tracking the current span accordingly.
  