"su" is different on Macs, instructions should reflect this

```
$ su -c "cd /home/archiveteam; git clone https://github.com/ArchiveTeam/{{REPO_NAME}}.git; cd {{REPO_NAME}}; ./get-wget-lua.sh" archiveteam
su: illegal option -- c
usage: su [-] [-flm] [login [args]]
```

probably an issue with the next line as well