message: Fix a broken link in `Threading and Queues` docs
  The link to Datasets in `Threading and Queues` seems
  to be broken. This fix attempts to fix the broken link.
diff: Note: In versions of TensorFlow before 1.2, we recommended using multi-threaded,
  queue-based input pipelines for performance. Beginning with TensorFlow 1.4,
  however, we recommend using the `tf.data` module instead. (See
  -[Datasets](datasets) for details. In TensorFlow 1.2 and 1.3, the module was
  +@{$datasets$Datasets} for details. In TensorFlow 1.2 and 1.3, the module was
  called `tf.contrib.data`.) The `tf.data` module offers an easier-to-use
  interface for constructing efficient input pipelines. Furthermore, we've stopped
  developing the old multi-threaded, queue-based input pipelines. We've retained
  