doc: use consistent terminology in process doc

`process.md` uses `POSIX` in most places, but `Unix` (and in one case
`Linux`) in a handful of cases where `POSIX` is appropriate. Change
those instances to `POSIX`.

PR-URL: nodejs/node#15321
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>