[docs] bad RST in PretrainedModel.generate docstring
![image](https://user-images.githubusercontent.com/6045025/80641266-96218b00-8a32-11ea-93d6-36484b3aad92.png)
https://github.com/huggingface/transformers/blob/master/src/transformers/modeling_utils.py#L857
fixed in Marian PR