assert: fix generatedMessage

The generatedMessage was wrong in case simple assert was used and
a message was auto generated. This fixed the TODO.

PR-URL: https://github.com/nodejs/node/pull/18322


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>