doc: add error check to fs example

Previously, the err passed to the callback of fs.open() was not checked.

PR-URL: https://github.com/nodejs/node/pull/18681


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>