Code example fix

Fixed like shown below. Also, stripped all unnecessary whitespace.

```javascript
var MySmartDescriptionModel = State.extend({
    description: 'string'; // Wrapped this into the `props` attribute.
    derived: {
        linkified: {
            deps: ['description'],
            fn: function () {
                return linkify(this.description);
            }
        }
    }
});
```

```javascript
var MySmartDescriptionModel = State.extend({
    props: {
      description: 'string';
    },
    derived: {
        linkified: {
            deps: ['description'],
            fn: function () {
                return linkify(this.description);
            }
        }
    }
});
```