doc: fix/improve inspector profiler example

The first parameter to the callback is `err`. Fix that. Expand example
to demonstrate an actual write to disk.

PR-URL: https://github.com/nodejs/node/pull/19379


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>