Fixed pagination example wrt new next_page_number behavior

    >>> page2.has_other_pages()

    True

    >>> page2.next_page_number()

    3

    Traceback (most recent call last):

    ...

    EmptyPage: That page contains no results

    >>> page2.previous_page_number()

    1

    >>> page2.start_index() # The 1-based index of the first item on this page