Just a small correction to the README

Hey guys, 

the packages works like a charm, thanks a lot for making TabPFN so easily accessible.

When I started with the README example, I stumbled across this: 

```
X_test = X[81:length(X),]
```

and I think you mean this: 

```
X_test = X[81:nrow(x),]
```

Feel free to adjust it 😉 

Best regards from Freiburg!