Fix reference to non-existing translation in the exports page. (mastodon#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 dcd8620 (PR mastodon#14956).

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

Author: shello

app/views/settings/exports/show.html.haml

@@ -39,7 +39,7 @@
       %tr
         %th= t('exports.bookmarks')
         %td= number_with_delimiter @export.total_bookmarks
-        %td= table_link_to 'download', t('bookmarks.csv'), settings_exports_bookmarks_path(format: :csv)
+        %td= table_link_to 'download', t('exports.csv'), settings_exports_bookmarks_path(format: :csv)
 
 %hr.spacer/