[PR] rename OS X to macOS
[PR] rename OS X to macOS

Summary:
Very minor thing, but OS X was renamed to macOS long time ago, good to update naming.
Pull Request resolved: https://github.com/facebook/flow/pull/7744

Differential Revision: D15421017

Pulled By: mroch

fbshipit-source-id: 475b274a5ea7d50b6ff58f66e94a57cf36fe3c3e

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