Change write function argument order

I had to correct your README edit when you added roi.write("output.tif", "float"), because pixel_type is third, it would produce an error.
But it is nicer and similar to the classic otbcli_ syntax, so we should adopt this.

New suggestion :

    def write(
        self,
        path: str | Path | dict[str, str] = None,
        pixel_type: dict[str, str] | str = None,
        preserve_dtype: bool = False,
        ext_fname: str = "",
        **kwargs
    ) -> bool:

Since ext_fname can also be passed in each filepath, we can have it as last argument.