doc: fix code snippets in tls.md

Replace `server.close()` which don't exist in code snippets.

PR-URL: https://github.com/nodejs/node/pull/23239


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>