tokio-rustls: Fix "Basic Structure of a Client" code in README

[jwodder]

The code shown in tokio-rustls's README under "Basic Structure of a Client" does not work with the current version of the library. Specifically, using tokio-rustls 0.24.0 and webpki 0.22.0, the following code:

use std::sync::Arc;
use tokio::net::{TcpStream, ToSocketAddrs};
use tokio_rustls::{rustls::ClientConfig, TlsConnector};
use webpki::DNSNameRef;

pub async fn tls_connect<A: ToSocketAddrs>(
    addr: A,
) -> std::io::Result<tokio_rustls::client::TlsStream<TcpStream>> {
    let mut config = ClientConfig::new();
    config
        .root_store
        .add_server_trust_anchors(&webpki_roots::TLS_SERVER_ROOTS);
    let config = TlsConnector::from(Arc::new(config));
    let dnsname = DNSNameRef::try_from_ascii_str("www.rust-lang.org").unwrap();
    let stream = TcpStream::connect(&addr).await?;
    config.connect(dnsname, stream).await
}

fails to build with the following errors (and warnings):

``` warning: use of deprecated type alias `webpki::DNSNameRef`: use DnsNameRef --> src/lib.rs:4:13 | 4 | use webpki::DNSNameRef; | ^^^^^^^^^^ | = note: `#[warn(deprecated)]` on by default warning: use of deprecated type alias `webpki::DNSNameRef`: use DnsNameRef --> src/lib.rs:14:19 | 14 | let dnsname = DNSNameRef::try_from_ascii_str("www.rust-lang.org").un... | ^^^^^^^^^^ error[E0599]: no function or associated item named `new` found for struct `ClientConfig` in the current scope --> src/lib.rs:9:36 | 9 | let mut config = ClientConfig::new(); | ^^^ function or associated item not found in `ClientConfig` error[E0308]: mismatched types --> src/lib.rs:16:20 | 16 | config.connect(dnsname, stream).await | ------- ^^^^^^^ expected enum `ServerName`, found struct `DnsNameRef` | | | arguments to this method are incorrect | note: associated function defined here --> /Users/jwodder/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-rustls-0.24.0/src/lib.rs:109:12 | 109 | pub fn connect(&self, domain: rustls::ServerName, stream: IO) -... | ^^^^^^^ ```

This PR updates the README to something that will compile, based on the client example program.

Closes #100.