Fix docs/staticfiles.md wrong signature

[NoPointExc]

The code:

class StaticFiles:
    def __init__(
        self,
        *,
        directory: typing.Optional[PathLike] = None,
        packages: typing.Optional[
            typing.List[typing.Union[str, typing.Tuple[str, str]]]
        ] = None,
        html: bool = False,
        check_dir: bool = True,
        follow_symlink: bool = False,
    ) -> None:

I found this while reading the doc: https://www.starlette.io/staticfiles/

[Kludex]

Thanks @NoPointExc 🙏