util: fix custom inspect description

Using a custom inspect function on the inspected object is
deprecated. Remove the reference from the option description
to make sure the user will read about the deprecation in the
more detailed description.

Backport-PR-URL: https://github.com/nodejs/node/pull/19230
PR-URL: https://github.com/nodejs/node/pull/17576


Reviewed-By: Anna Henningsen <anna@addaleax.net>