message: remove "usage example" text, fix indentation
diff: @@ -1875,7 +1875,6 @@ def rgb_to_grayscale(images, name=None):
  Returns:
  The converted grayscale image(s).
  - Usage Example:
  ```python
  >>> original = tf.constant([[[1.0], [2.0], [3.0]]])
  >>> converted = tf.image.rgb_to_grayscale(original)
  @@ -1912,7 +1911,6 @@ def grayscale_to_rgb(images, name=None):
  Returns:
  The converted grayscale image(s).
  - Usage Example:
  ```python
  >>> original = tf.constant([[[1.0, 2.0, 3.0]]])
  >>> converted = tf.image.grayscale_to_rgb(original)
  