Misguidance of onnx export in the README.md

[L-M-Sherlock]

I find that there is not kwarg named backend in the RWKV(), so I replace backend with mode.

Before:

from rwkvstic.load import RWKV
from rwkvstic.agnostic.backends import ONNX_EXPORT
import torch

model = RWKV("model.pth", backend=ONNX_EXPORT, dtype=torch.float16) # or torch.float32
# the model is exported to model_{layers}_{embed}.onnx
# the external data is stored in model_{layers}_{embed}.bin

After

from rwkvstic.load import RWKV
from rwkvstic.agnostic.backends import ONNX_EXPORT
import torch

model = RWKV("model.pth", mode=ONNX_EXPORT, dtype=torch.float16) # or torch.float32
# the model is exported to model_{layers}_{embed}.onnx
# the external data is stored in model_{layers}_{embed}.bin