doc: change Mac OS X to macOS

This update changes references to "Mac OS X", "OS X", and "OSX" in
markdown files to "macOS".

PR-URL: https://github.com/nodejs/node/pull/12106
Fixes: https://github.com/nodejs/node/issues/12086


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>