doc: update examples in api/crypto.md

* var -> const / let in crypto.md
* fix error in crypto.md code example
* equal -> strictEqual, == -> === in crypto.md
* update estimated outputs in crypto.md
* snake_case -> camelCase in crypto.md examples
* concatenation -> multiline template in crypto
* add missing line break in crypto code example
* add missing link reference in crypto.md

PR-URL: https://github.com/nodejs/node/pull/10909


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>