doc: use sentence case in headers in src/crypto/README.md

Use sentence case consistently in headers in src/crypto/README.md.

PR-URL: https://github.com/nodejs/node/pull/38524


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>