Minor tweak to /etc/sudoers

Was working on a vagrant box and wanted to execute a command as a user other than vagrant or root.

With the existing documentation on the /etc/sudoers entry it's necessary to do this for example (contrived as all get out):
```sudo su graphite -c "touch /var/log/carbon/carbon.log"```
With the change it becomes:
```sudo -u graphite touch /var/log/carbon/carbon.log```

It shouldn't be necessary to become root, to execute su to become another user, to execute a command as that user.
We should just be able to execute a command as another user

The (ALL) allows us to become any user, without it, sudo defaults to only allowing root.
This change shouldn't break anything for anyone.