Combining streams recipe doesn't work

[Combining streams to handle errors](https://github.com/gulpjs/gulp/blob/master/docs/recipes/combining-streams-to-handle-errors.md)

The line `combined.on('error', function(err) {...});` just throws an error:

```
TypeError: Object function (){
    return pipeline.apply(pipeline, args);
  } has no method 'on'
```
