ips is undefined

Using the second example from the docs throws the error
**TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined`**
```javascript
cloudflareIPs((err, ips) => {
app.set('trust proxy', ['loopback', ...ips]);
});
```

The callback with 2 params doesn't work as there is no `err` passed. I think easiest solution is to just remove this callback style from the docs.

Fix: remove `err` from the callback.
```javascript
cloudflareIPs((ips) => {
app.set('trust proxy', ['loopback', ...ips]);
});
```

Start with the second callback example in the documentation, which currently declares err and leaves ips undefined. Update the example to use the single callback parameter shown in the issue, then verify that the documented spread expression no longer throws the reported TypeError.

Written by the indexing model from the issue text.