Update README.md - correction to bootstrap example

Current bootstrap example produces 
```html
<label for="id_<label for=" id_username"="" class="control-label col col-lg-3">Username</label>
```

When ```label``` is changed to ...
```javascript
var label = object.labelHTML(name);
```

The resulting html is correct...
```html
<label for="id_username" class="control-label col col-lg-3">Username</label>
```