Updated rights language

I have updated the rights language in the TEI/XML of all items in the
following sections:

1. Published Works --> Books by Whitman --> U.S. Editions of Leaves of Grass
2. Published Works --> Periodicals --> Whitman's Poems in Periodicals
3. Published Works --> Periodicals --> Journalism

At present, this change is visible only in the TEI/XML made available
from each page. At a later stage, we will likely expose the rights
language in the HTML view.

--Liz