add missing offset in iter_chunked

[honglei]

iter_chunked will always start with offset=0, the seek func not work.

from aiofile import async_open

async def test(path: str, restart_offset: int):
    BLOCK_SIZE = 8192
    async with async_open(
                    path, "rb"
        ) as file_out:
        file_out.seek(restart_offset)
        async for chunk in file_out.iter_chunked(BLOCK_SIZE):
            #pass