doc: fix typo in doc/api/process.md

PR-URL: https://github.com/nodejs/node/pull/12612
Fixes: https://github.com/nodejs/node/issues/12565


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>