Fix VierteljahresNachweis export error - correct method name

🐛 Error Fixed:
- AttributeError: 'VierteljahresNachweis' object has no attribute 'completion_percentage'
- Line 5347 in destinataer_export view was calling completion_percentage()
- Corrected to call get_completion_percentage() which is the actual method name

✅ Export functionality now works correctly for destinataers with quarterly confirmations

The error occurred when exporting a destinataer that had quarterly tracking data.
All quarterly confirmation completion percentages are now properly exported.

app/stiftung/views.py

@@ -5344,7 +5344,7 @@ def destinataer_export(request, pk):
                     "geprueft_am": confirmation.geprueft_am.isoformat() if confirmation.geprueft_am else None,
                     "geprueft_von": confirmation.geprueft_von.username if confirmation.geprueft_von else None,
                     "faelligkeitsdatum": confirmation.faelligkeitsdatum.isoformat() if confirmation.faelligkeitsdatum else None,
-                    "completion_percentage": confirmation.completion_percentage(),
+                    "completion_percentage": confirmation.get_completion_percentage(),
                     "uploaded_files": []
                 }