Inconsistent capitalization for BitnetForCausalLM caused error in /utils/convert-hf-to-gguf-bitnet.py

Caused #178 most likely

Code was looking for `BitNetForCausalLM`, name in `convert-hf-to-gguf-bitnet.py` was:
 
https://github.com/microsoft/BitNet/blob/fd9f1d6e46b476d449417d49851f50a569165835/utils/convert-hf-to-gguf-bitnet.py#L952

`BitNetForCausalLM` vs.

`BitnetForCausalLM`

