doc: fix documentation of process.argv

The current documentation states that if run something like
`node app.js` then in our process.argv array first elements is `node`,
but actually it's `process.execPath` not `node`
as documentation currently suggests.

Fixes: https://github.com/nodejs/node/issues/7434
PR-URL: https://github.com/nodejs/node/pull/7449


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>