Fix reference to non-existing translation in the exports page. (#15894)

The exports page showed a different "CSV" capitalisation in the
"Bookmarks" row ("Csv") compared to the other rows ("CSV").
This was due to a referece to a translation string that does not exist,
`bookmarks.csv`, defaulting to the key's last segment in title case.

This issue was introduced in commit dcd86204 (PR #14956).

(h/t @meqif for helping with figuring out the bug)