doc: fix https.timeout docs

`server.timeout` is a property, not a method

PR-URL: https://github.com/nodejs/node/pull/12039


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>