message: Fix misformatted markdown in Model.fit docstring
  The current documentation of `tf.keras.models.Model.fit` contains misformatted markdown in the description of the `validation_data` attribute: it is missing a line break after the list enumerating the possible types of accepted arguments, which hurts legibility upon rendering.
  See [keras documentation](https://www.tensorflow.org/api_docs/python/tf/keras/Model#fit).
  This commit fixes that problem.
diff: @@ -676,6 +676,7 @@ class Model(network.Network, version_utils.VersionSelector):
  - tuple `(x_val, y_val)` of Numpy arrays or tensors
  - tuple `(x_val, y_val, val_sample_weights)` of Numpy arrays
  - dataset
  +
  For the first two cases, `batch_size` must be provided.
  For the last case, `validation_steps` could be provided.
  shuffle: Boolean (whether to shuffle the training data
  