doc: make YAML matter consistent in crypto.md

This fixes YAML that gets incorrectly processed by our tooling.

Refs: https://github.com/nodejs/node/pull/29910#issuecomment-543237767

PR-URL: https://github.com/nodejs/node/pull/30016


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>