change os name: Mac OS to macOS (zsh-users#656)

README.md

@@ -20,7 +20,7 @@ zsh-completions ![GitHub release](https://img.shields.io/github/release/zsh-user
 | NixOS | [zsh-completions](https://github.com/NixOS/nixpkgs/blob/master/pkgs/shells/zsh/zsh-completions/default.nix) |
 | Void Linux | [zsh-completions](https://github.com/void-linux/void-packages/blob/master/srcpkgs/zsh-completions/template) |
 | Slackware | [Slackbuilds](https://slackbuilds.org/repository/14.2/system/zsh-completions/) |
-| Mac OS | [homebrew](https://github.com/Homebrew/homebrew-core/blob/master/Formula/zsh-completions.rb), [MacPorts](https://github.com/macports/macports-ports/blob/master/sysutils/zsh-completions/Portfile)  |
+| macOS | [homebrew](https://github.com/Homebrew/homebrew-core/blob/master/Formula/zsh-completions.rb), [MacPorts](https://github.com/macports/macports-ports/blob/master/sysutils/zsh-completions/Portfile)  |
 | NetBSD | [pkgsrc](http://ftp.netbsd.org/pub/pkgsrc/current/pkgsrc/shells/zsh-completions/README.html)  |