Update docs for parameter rename

OpenAIGPTLMHeadModel now accepts `labels` instead of `lm_labels`

pytorch_transformers/modeling_openai.py

@@ -538,7 +538,7 @@ class OpenAIGPTLMHeadModel(OpenAIGPTPreTrainedModel):
     r"""
         **labels**: (`optional`) ``torch.LongTensor`` of shape ``(batch_size, sequence_length)``:
             Labels for language modeling.
-            Note that the labels **are shifted** inside the model, i.e. you can set ``lm_labels = input_ids``
+            Note that the labels **are shifted** inside the model, i.e. you can set ``labels = input_ids``
             Indices are selected in ``[-1, 0, ..., config.vocab_size]``
             All labels set to ``-1`` are ignored (masked), the loss is only
             computed for labels in ``[0, ..., config.vocab_size]``