Fix typograpical errors (#2438)

src/transformers/modeling_transfo_xl.py

@@ -449,7 +449,7 @@ class AdaptiveEmbedding(nn.Module):
 
 class TransfoXLPreTrainedModel(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 = TransfoXLConfig