fix wrong path to ReduceLROnPlateau in docstring

Author: quanvuong

torch/optim/lr_scheduler.py

@@ -197,7 +197,7 @@ class ReduceLROnPlateau(object):
 
  Example:
  >>> optimizer = torch.optim.SGD(model.parameters(), lr=0.1, momentum=0.9)
- >>> scheduler = torch.optim.ReduceLROnPlateau(optimizer, 'min')
+ >>> scheduler = ReduceLROnPlateau(optimizer, 'min')
  >>> for epoch in range(10):
  >>> train(...)
  >>> val_loss = validate(...)