Update and rename on-mac-os-x.md to on-macOS.md

[str8outtajc]

Change references to Mac from "OS X" to "macOS"

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[mbushkov]

This PR renames a file that is referenced here:

grr-doc/deploying-grr-clients/index.md

Line 6 in b54abba

* [On Mac OS X](on-mac-os-x.md)

deploying-grr-clients/index.md should be changed accordingly, otherwise we'll have a broken link.

[panhania]

While I agree with the overall changes to this file, I don't think we should use capitalization in the filename. It looks awkward and inconsistent.