doc: fix and modernize code examples in crypto.md

[vsemozhetbyt]

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, crypto

• var -> const / let.
• Fix error in code example (basing on the previous example).
• equal -> strictEqual, == -> ===.
• Update estimated outputs in code example comments.
• snake_case -> camelCase (unlike method signatures, examples mostly use camelCase).
• Concatenation -> multiline template.
• Add missing line break in code example (the code throws currently; fixed basing on the history).
• Add missing link reference.