Do not quote 0.0.0.0 in quickstart docs

It brakes when used in systemd unit file directly like:

```
ExecStart=/usr/bin/docker run --name vulcand -p 8182:8182 -p 8181:8181 mailgun/vulcand /go/bin/vulcand -apiInterface="0.0.0.0" --etcd=http://172.17.42.1:4001
```

This is because systemd executes the command without bash hance no quote parsing and vulcand raises error:

```
Service exited with error: service start failure: listen tcp: lookup "0.0.0.0": no such host
```