refine code by commment

Author: zhangbo9674

python/paddle/optimizer/optimizer.py

@@ -1047,7 +1047,7 @@ def clear_grad(self, set_to_zero=True):
  There are two method to clear grad: set_to_zero or delete grad.
  
  Args:
- set_to_zero (bool): If set grads to zero or not, default is True.
+ set_to_zero (bool, optional): If set grads to zero or not, default is True.
  
  Returns:
  None