Fixing typo in gpt2 for doc site's class link

pytorch_transformers/modeling_gpt2.py

@@ -290,7 +290,7 @@ GPT2_INPUTS_DOCSTRING = r"""    Inputs:
             Indices of input sequence tokens in the vocabulary.
             GPT-2 is a model with absolute position embeddings so it's usually advised to pad the inputs on
             the right rather than the left.
-            Indices can be obtained using :class:`pytorch_transformers.BPT2Tokenizer`.
+            Indices can be obtained using :class:`pytorch_transformers.GPT2Tokenizer`.
             See :func:`pytorch_transformers.PreTrainedTokenizer.encode` and
             :func:`pytorch_transformers.PreTrainedTokenizer.convert_tokens_to_ids` for details.
         **past**: