doc: add OS X to instead of only macOS

The last OS X version is 10.11 El Captian, and the first macOS version
is 10.12 Sierra. So >= 10.8 < 10.10 is called OS X and >= 10.10 is
called OS X or macOS.

Refs: https://en.wikipedia.org/wiki/MacOS#Release_history

PR-URL: https://github.com/nodejs/node/pull/21033


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>