Fix model type in example

Use `gpt2` scripts instead of `bert` as model is GPT2 based.

README.md

@@ -79,7 +79,7 @@ you need to tell python where to find the clone of Megatron-LM, e.g.:
 ```
 cd /tmp
 git clone https://github.com/NVIDIA/Megatron-LM
-PYTHONPATH=/tmp/Megatron-LM python src/transformers/models/megatron_bert/convert_megatron_bert_checkpoint.py ...
+PYTHONPATH=/tmp/Megatron-LM python src/transformers/models/megatron_gpt2/convert_megatron_gpt2_checkpoint.py ...
 ```
 Or, if you already have it cloned elsewhere, simply adjust the path to the existing path.