doc: fix process.stdout fd number

it should be 1 for the stdout fd number

PR-URL: https://github.com/nodejs/node/pull/12055


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>