doc: fix a typo in api/process.md

Fix a mistyped module name in example REPL sessions found in the
description of the 'warning' event: it should be `events` instead of
`event`.

PR-URL: https://github.com/nodejs/node/pull/11780


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>