message: Change example snippet to DocTest format
diff: @@ -100,14 +100,15 @@ def elu(x, alpha=1.0):
  Example Usage:
  - ```python3
  - model = models.Sequential()
  - model.add(layers.Conv2D(32, (3, 3), activation='elu', input_shape=(28, 28, 1)))
  - model.add(layers.MaxPooling2D((2, 2)))
  - model.add(layers.Conv2D(64, (3, 3), activation='elu'))
  - model.add(layers.MaxPooling2D((2, 2)))
  - model.add(layers.Conv2D(64, (3, 3), activation='elu'))
  - ```
  +>>> from tensorflow.keras import models, layers
  +>>> model = models.Sequential()
  +>>> model.add(layers.Conv2D(32, (3, 3), activation='elu', input_shape=(28, 28, 1)))
  +>>> model.add(layers.MaxPooling2D((2, 2)))
  +>>> model.add(layers.Conv2D(64, (3, 3), activation='elu'))
  +>>> model.add(layers.MaxPooling2D((2, 2)))
  +>>> model.add(layers.Conv2D(64, (3, 3), activation='elu'))
  +>>> model
  +<tensorflow.python.keras.engine.sequential.Sequential object ...>
  Arguments:
  x: Input tensor.
  