doc: use sentence case in README.md headers

PR-URL: https://github.com/nodejs/node/pull/37645


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>