doc: fix .mjs syntax in crypto.md

Co-authored-by: Antoine du Hamel <duhamelantoine1995@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/38882


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>