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>

doc/api/errors.md

@@ -523,7 +523,7 @@ found [here][online].
   [file descriptors][] allowable on the system has been reached, and
   requests for another descriptor cannot be fulfilled until at least one
   has been closed. This is encountered when opening many files at once in
-  parallel, especially on systems (in particular, OS X) where there is a low
+  parallel, especially on systems (in particular, macOS) where there is a low
   file descriptor limit for processes. To remedy a low limit, run
   `ulimit -n 2048` in the same shell that will run the Node.js process.