Fix typograpical errors (#2438)

src/transformers/modeling_ctrl.py

@@ -163,7 +163,7 @@ class EncoderLayer(torch.nn.Module):
 
 class CTRLPreTrainedModel(PreTrainedModel):
     """ An abstract class to handle weights initialization and
-        a simple interface for dowloading and loading pretrained models.
+        a simple interface for downloading and loading pretrained models.
     """
 
     config_class = CTRLConfig