Removed public="false" from the service block example

Since this service must be public otherwise you can't use it when you would like to render it like this:
        
        $blockHelper = $this->get('sonata.block.templating.helper');
        $block = $blockHelper->render(array(
                'type'      => 'sonata.block.service.text',
                'settings'  => array(
                        'content'   => 'My content'
                    )
            )
        );