Fix README: `Llama3VisionAlpha` not `Llama3VisionAlphaChatHandler`

Under "Multi-modal Models", [README.md](https://github.com/abetlen/llama-cpp-python/blob/main/README.md?plain=1#L511) says to use `Llama3VisionAlphaChatHandler`. However in [llama_cpp/llama_chat_format.py](https://github.com/abetlen/llama-cpp-python/blob/main/llama_cpp/llama_chat_format.py#L3101) we see the correct name is `Llama3VisionAlpha`.