l10n: de.po: Fix typo in German translation

l10n: de.po: Fix typo in German translation

Fix translation error of "complete => "vollständig" instead of
"unvollständig"

Currently: Documentation states that --unshallow can NOT be used on
INcomplete projects. This is wrong;
Correct would be: --unshallow can NOT be used on complete projects.

This change fixes that error in the German translation.

Signed-off-by: Philipp Weißmann <mail@philipp-weissmann.de>
Reviewed-by: Matthias Rüster <matthias.ruester@gmail.com>