Update llama README.md

Update FullModelHFCheckpointer import to align with TorchTune API changes.  

https://github.com/pytorch/torchtune/blob/7d29c21149ace4be9fdc2709a03c0a21ba191010/torchtune/training/__init__.py#L41

Author: JamePeng

examples/models/llama/README.md

@@ -232,7 +232,7 @@ You can also download above models from [Hugging Face](https://huggingface.co/).
 
 
 ```Python
-from torchtune.utils import FullModelHFCheckpointer
+from torchtune.training import FullModelHFCheckpointer
 from torchtune.models import convert_weights
 import torch