message: Add a usage example for math.sigmoid
diff: @@ -3253,6 +3253,14 @@ def sigmoid(x, name=None):
  Specifically, `y = 1 / (1 + exp(-x))`.
  + Example Usage:
  +
  + >>> x = tf.constant([0.0, 0.2, 0.3, 0.5, 0.7, 1.0])
  + >>> tf.math.sigmoid(x)
  + <tf.Tensor: shape=(6,), dtype=float32, numpy=
  + array([0.5 , 0.54983395, 0.5744425 , 0.62245935, 0.66818774,
  + 0.7310586 ], dtype=float32)>
  +
  Args:
  x: A Tensor with type `float16`, `float32`, `float64`, `complex64`, or
  `complex128`.
  