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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines