docs(example/tcp_echo): fix TCP echo leaking resources (#8997)

docs/examples/tcp_echo.md

@@ -19,7 +19,7 @@ returns to the client anything it sends.
 const listener = Deno.listen({ port: 8080 });
 console.log("listening on 0.0.0.0:8080");
 for await (const conn of listener) {
-  Deno.copy(conn, conn);
+  Deno.copy(conn, conn).finally(() => conn.close());
 }
 ```