Fix cargo clippy issue

src/tcp_relay_client/relay_client.rs

@@ -74,7 +74,7 @@ async fn relay_connection(ws_url: String, socket: TcpStream, conf: Arc<ClientCon
                     .expect("Failed to parse client auth private key!");
 
                 config
-                    .with_single_cert(certs, key)
+                    .with_client_auth_cert(certs, key)
                     .expect("Failed to set client certificate!")
             }
         };