message: core: remove `Current::unknown` from docs
  Currently, the `tracing-core` docs for `Collect` reference the
  `Current::unknown` function, which doesn't exist in 0.2. This means
  there's a broken link in the docs.
  This commit fixes that.
diff: @@ -403,12 +403,8 @@ pub trait Collect: 'static {
  /// if the thread from which this method is called is inside a span,
  /// or [`Current::none`] if the thread is not inside a span.
  ///
  - /// If the collector does not implement a current span, it should
  - /// return [`Current:unknown`].
  - ///
  /// [`Current::new`]: super::span::Current::new
  /// [`Current::none`]: super::span::Current::none
  - /// [`Current::unknown`]: super::span::Current::unknown
  fn current_span(&self) -> span::Current;
  // === Downcasting methods ================================================
  