doc: use sentence-case in quic.md headers

For consistency with the rest of our docs and our style guide, use
sentence-case rather than headline-case in the headers in quic.md.

PR-URL: https://github.com/nodejs/node/pull/34453


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>