Fixed Event_Instance::unregister()

The annotation in 2nd param is <code>null for all</code>.
```php
	/**
	 * Unregister/remove one or all callbacks from event
	 *
	 * @param   string   $event     event to remove from
	 * @param   mixed    $callback  callback to remove [optional, null for all]
	 * @return  boolean  wether one or all callbacks have been removed
	 */
```

But in condition is <code>true</code>