To make a string in baobab's vala source translatable

[Andika Triwidada <andika gmail com>]

Hello,

While testing translation of baobab, I found a string that should have
been translatable but not yet.
That string resides in a vala code [1].
Would anyone comment/review my patch? It was to make that string translatable.
I have tested it after applying to my test box and rebuilt.

Regards,
Andika Triwidada

1. https://bugzilla.gnome.org/show_bug.cgi?id=679785