fix typo

cc @thomwolf

pytorch_pretrained_bert/modeling_gpt2.py

@@ -680,7 +680,7 @@ class GPT2DoubleHeadsModel(GPT2PreTrainedModel):
 
     config = modeling_gpt2.GPT2Config()
 
-    model = modeling_gpt2.GPT2LMHeadModel(config)
+    model = modeling_gpt2.GPT2DoubleHeadsModel(config)
     lm_logits, multiple_choice_logits, presents = model(input_ids, mc_token_ids)
     ```
     """

pytorch_pretrained_bert/modeling_openai.py

@@ -789,7 +789,7 @@ class OpenAIGPTDoubleHeadsModel(OpenAIGPTPreTrainedModel):
 
     config = modeling_openai.OpenAIGPTConfig()
 
-    model = modeling_openai.OpenAIGPTLMHeadModel(config)
+    model = modeling_openai.OpenAIGPTDoubleHeadsModel(config)
     lm_logits, multiple_choice_logits = model(input_ids, mc_token_ids)
     ```
     """