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);
            }
        }
    }
});
```