message: Update tf.keras.backend.cast_to_floatx example
  Redoing this PR against master instead of 2.0.
diff: @@ -158,7 +158,7 @@ def cast_to_floatx(x):
  Example:
  ```python
  - >>> from keras import backend as K
  + >>> from tensorflow.keras import backend as K
  >>> K.floatx()
  'float32'
  >>> arr = numpy.array([1.0, 2.0], dtype='float64')
  