Fix example output of PaginatorHelper::prev()

The docs show that a `<span>` tag is output by default, but the [current](https://github.com/cakephp/cakephp/blob/1cb44d4ba4675332248bc53af2729c5083529374/src/View/Helper/PaginatorHelper.php#L70) version of the helper uses an `<a>` tag instead.