fix typo in translatable string

Change-Id: Idf13d1dd35811a94100aa824f310006c2fd1b82f
Reviewed-on: https://gerrit.libreoffice.org/13467
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>