doc: update exercise portion of onboarding doc

PR-URL: https://github.com/nodejs/node/pull/8559


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>