Update the README to include the correct module code.

[mgermaine93]

Hi Cheukting,

Thanks so much for hosting the "Writing Python Modules in Rust" tutorial at PyCon 2024! I was able to follow along in the README for the most part, but ran into trouble whenever I needed to add the module code in lib.rs, specifically the parts like this (I kept getting "Not Found in this Scope" errors):

/// A Python module implemented in Rust.
#[pymodule]
fn pyo3_101(m: &Bound<'_, PyModule>) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(say_hello, m)?)?;
    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
    Ok(())
}

After talking with someone else in the audience, I found that this code worked instead for me:

/// A Python module implemented in Rust.
#[pymodule]
fn pyo3_101(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;
    m.add_function(wrap_pyfunction!(say_hello, m)?)?;
    Ok(())
}

Not sure if other people have had the same issue. I'm brand new to Rust, so hopefully this helps more people get up and running.

Thanks again!

~Matt