src: fix minor typo in comments

Correcting a couple of minor spelling typos in comments.

PR-URL: https://github.com/nodejs/node/pull/8736


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>