Bug 28427: Terminology: Shelf should be list

Bug 28427: Terminology: Shelf should be list

This patch corrects the title element on the download list page in the
staff interface.

The affected page should never be seen by the user because other
error-handling should take precedence. In order to see it you have to
set up such an error:

- Log into the staff client as a user with existing lists.
- Open the Lists page.
- Open one of your lists in a new tab.
- In the original tab, delete the list you opened.
- In the tab showing the contents of the list, click the "Download" menu
  button and select an option.
- You should be redirected to a page with an error message at the top.
- The title of this page should be correct: "Download list."

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>