Typo corrections for German language file.

Small typo corrections for the German language file.

Notice: Link in header:

```bash
Basic guidelines for this translation: http://i18n.xfce.org/wiki/team_de
```

is outdated (does not exist) -> maybe it can be replaced with this hyperlink:

```bash
http://wiki.xfce.org/de/translations#das_team
```

Stefan :)