message: docs: fix typo in tracing-subscriber example
diff: @@ -33,7 +33,7 @@ use tracing_log::NormalizeEvent;
  /// "level":"INFO",
  /// "fields":{"message":"some message","key":"value"}
  /// "target":"mycrate",
  -/// "span":{name":"leaf"},
  +/// "span":{"name":"leaf"},
  /// "spans":[{"name":"root"},{"name":"leaf"}],
  /// }
  /// ```
  