doc: improve instructions for verifying binaries

Simplify and clarify the text in README.md for verifying binaries.

PR-URL: https://github.com/nodejs/node/pull/23248


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>