message: Update gradient_descent.py
  Typing mistake in documentation.
diff: @@ -40,7 +40,7 @@ class SGD(optimizer_v2.OptimizerV2):
  ```python
  velocity = momentum * velocity - learning_rate * g
  - w = w * velocity
  + w = w + velocity
  ```
  When `nesterov=False`, this rule becomes:
  