[balsa] Fix Vietnamese translation of default:mm

[Piotr Drąg <piotrdrag src gnome org>]

commit 68d63d8104622087fcc638b0acdfc3d1dc88a9a4
Author: Piotr Drąg <piotrdrag gmail com>
Date:   Sat Sep 26 16:00:07 2015 +0200

    Fix Vietnamese translation of default:mm

 po/vi.po |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/po/vi.po b/po/vi.po
index a1e77fc..15f2a22 100644
--- a/po/vi.po
+++ b/po/vi.po
@@ -5695,7 +5695,7 @@ msgstr "Không thể in trang %d vì tài liệu này chỉ có %d trang."
 #. * it isn't default:mm or default:inch it will not work
 #.
 msgid "default:mm"
-msgstr "mm"
+msgstr "default:mm"
 
 #: ../src/print-gtk.c:435
 msgid "inch"