Added doc comment to STS credentials-provider code example (#513)

use std::time::{Duration, SystemTime};

use sts::Credentials;

/// Implements a basic version of ProvideCredentials with AWS STS

/// and lists the tables in the region based on those credentials.

#[tokio::main]

async fn main() -> Result<(), dynamodb::Error> {

    tracing_subscriber::fmt::init();

    Ok(())

}

/// This is a rough example of how you could implement ProvideCredentials with Sts

/// This is a rough example of how you could implement ProvideCredentials with Amazon STS.

///

/// Do not use this in production! A high quality implementation is in the roadmap.

#[derive(Clone)]