Fixed group tag name for printer object

As per [CUPS ipptoolfile documentation](http://www.cups.org/documentation.php/doc-1.7/man-ipptoolfile.html):

### Tags
Value and group tags correspond to the names from RFC 2911 and other IPP extension specifications. Here are  the group tags:
```
    event-notification-attributes-tag
    job-attributes-tag
    operation-attributes-tag
    printer-attributes-tag
    subscription-attributes-tag
    unsupported-attributes-tag
```