lib/scatterlist: Remove leftover from sg_page_iter comment

Commit d901b276 ("lib/scatterlist: Provide a DMA page iterator") added
the sg DMA iterator but a leftover remained in the sg_page_iter
documentation as you cannot get the page dma address (only the page
itself), fix it.

Signed-off-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>