Fix subtask viewset typo error

Author: Sosokker

backend/tasks/tasks/views.py

@@ -177,7 +177,7 @@ def destroy(self, request, *args, **kwargs):
     def partial_update(self, request, *args, **kwargs):
         """Update a subtask."""
         try:
-            instance = self.get_o
+            instance = self.get_object()
             serializer = self.get_serializer(instance, data=request.data, partial=True)
             serializer.is_valid(raise_exception=True)
             self.perform_update(serializer)