Update example in readme

[SimoMoilanen]

Hi, the example on how to use Equity Calculator didn't work when I tried it. I made a couple of changes and this is how I got it to work:

use rust_poker::hand_range::{HandRange, get_card_mask};
use rust_poker::equity_calculator::approx_equity;

let ranges = HandRange::from_strings(["AK,22+".to_string(), "random".to_string()].to_vec());
let public_cards = get_card_mask("2h3d4c");
let stdev_target = 0.01;
let n_threads = 4;
let result = approx_equity(&ranges, public_cards, n_threads, stdev_target);
match result {
    Ok(equity) => { println!("player 1 equity: {}", equity[0]) }
    Err(error) => { println!("{}", error) }
};

I tried to open a PR, but push was blocked.