doc: modernize code examples in the cluster.md

- Fixes https://github.com/nodejs/node/issues/10255
- It also will be consistent with a previous code example.
- `cluster.workers` iteration: `Object.keys().forEach` -> `for`...`in`

PR-URL: https://github.com/nodejs/node/pull/10270


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>