doc: update fs.open() changes record for optional 'flags'

Was missed on original PR.

Ref: https://github.com/nodejs/node/pull/23767

PR-URL: https://github.com/nodejs/node/pull/24240
Refs: https://github.com/nodejs/node/pull/23767


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>