l10n: de.po: use "das Tag" instead of "der Tag"

Use "das Tag" to avoid confusion with the German word "Tag" (day).

Reported-by: Dirk Heinrichs <dirk.heinrichs@altum.de>
Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Merge branch 'maint' of git://github.com/git-l10n/git-po into maint

* 'maint' of git://github.com/git-l10n/git-po:
  l10n: de.po: use "das Tag" instead of "der Tag"