doc: fix confusing example in process.md

PR-URL: https://github.com/nodejs/node/pull/12282
Fixes: https://github.com/nodejs/node/issues/12280


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>