update README sample code

[fukusuket]

I tweaked the sample code in the README a bit so it compiles.
Would be great if you could take a look. If the original sample was intentional, feel free to just close this PR.
Thank you for your time :)

In my environment, the following code compiled successfully.

[package]
name = "sample"
version = "0.1.0"
edition = "2024"

[dependencies]
sigmars = "0.2.2"
serde_json = "*"
tokio = "1.44.2"

As a collection of simple detections:

use std::error::Error;
use serde_json::json;
use sigmars::SigmaCollection;

fn main() -> Result<(), Box<dyn Error>> {
    let rules: SigmaCollection = SigmaCollection::new_from_dir("/path/to/sigma/rules/").unwrap();
    let event = json!({"foo": "bar"});
    let matches = rules.get_detection_matches(&event.into());
    
    Ok(())
}

or with correlations (requires tokio) using an in-memory backend

use std::error::Error;
use serde_json::json;
use tokio;
use sigmars::{MemBackend, SigmaCollection};

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
    let mut rules: SigmaCollection = SigmaCollection::new_from_dir("/path/to/sigma/rules/").unwrap();

    let mut backend = MemBackend::new().await;
    rules.init(&mut backend).await;

    let event = json!({"foo": "bar"});
    let matches = rules.get_matches(&event.into()).await?;
    
    Ok(())
}