[6.x] DocBlock: Grammar fix in Paginator Contract

[BrandonSurowiec]

Noticed the Paginator Contract incorrectly uses "is", while the implementation correctly uses "are."

    /**
     * Determine if there is more items in the data store.
     *
     * @return bool
     */
    public function hasMorePages();

    /**
     * Determine if there are more items in the data source.
     *
     * @return bool
     */
    public function hasMorePages()
    {
        return $this->currentPage() < $this->lastPage();
    }

[driesvints]

Thanks!