message: Use `get_cosine_decay_fn` to match the description
diff: @@ -65,7 +65,7 @@ class PowerSignOptimizer(optimizer.Optimizer):
  Example usage for PowerSign-cd (PowerSign with cosine sign decay)
  ```
  decay_steps = 1000
  - linear_decay_fn = sign_decays.get_linear_decay_fn(decay_steps)
  + linear_decay_fn = sign_decays.get_cosine_decay_fn(decay_steps)
  opt = PowerSignOptimizer(learning_rate=0.1, sign_decay_fn=linear_decay_fn)
  ```
  