doc: /s/SHASUM256/SHASUMS256

As an example, `curl https://nodejs.org/dist/v8.4.0/SHASUM256.txt` will
return a 404 right now.

PR-URL: https://github.com/nodejs/node/pull/15101


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>