doc: fix code type of markdowns

1. correct code type in addons.markdown
2. add missed code type in crypto.markdown

PR-URL: https://github.com/nodejs/node/pull/4858


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>