doc: rename README section for Release Keys

The section of the README currently labeled "Release Team" isn't about
the team much and is very much about the keys. Rename it "Release Keys"
so that it's purpose matches it's name.

PR-URL: https://github.com/nodejs/node/pull/23927


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>