doc: modernize child_process example code

1. equal => strictEqual.
2. let => const for the variable that is not reassigned.
3. fix spaces.
4. stringify erroneous raw buffer outputs.
5. fix a typo.

PR-URL: https://github.com/nodejs/node/pull/10102


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>