fdo#78332: sw: fix separators in SwDDEFieldType::PutValue()

Setting 3 separators instead of 2 does not work.

(regression from 263153842741d7ce21cc0bf1c5296a55a1138024)

Change-Id: Id0b4649404b8a49a7f89a237c46106bce4e146ff
(cherry picked from commit 747bcdc14386978f2a98a452e52c8dcc2459d319)
Reviewed-on: https://gerrit.libreoffice.org/9513
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>