Fix return type in FormElement#__invoke() docblock
Fix return type in FormElement#__invoke() docblock

Since Zend\Form\View\Helper\FormElement#render() always returns a string, __invoke() must return a string too.
MadCat34/zend-form,akrabat/zend-form,MadCat34/zend-form,zendframework/zend-form,akrabat/zend-form,zendframework/zend-form