message: Update python example for keras backend dtype
diff: @@ -956,10 +956,10 @@ def dtype(x):
  # Keras variable
  >>> kvar = K.variable(np.array([[1, 2], [3, 4]]))
  >>> K.dtype(kvar)
  - 'float32_ref'
  + 'float32'
  >>> kvar = K.variable(np.array([[1, 2], [3, 4]]), dtype='float32')
  >>> K.dtype(kvar)
  - 'float32_ref'
  + 'float32'
  ```
  """
  return x.dtype.base_dtype.name
  