Fix ValueError in chord with single task header (#3608)

celery/canvas.py

@@ -1242,7 +1242,7 @@ class chord(Signature):
         if len(self.tasks) == 1:
             # chord([A], B) can be optimized as A | B
             # - Issue #3323
-            return (self.tasks[0].set(task_id=task_id) | body).apply_async(
+            return (self.tasks[0] | body).set(task_id=task_id).apply_async(
                 args, kwargs, **options)
         # chord([A, B, ...], C)
         return self.run(tasks, body, args, task_id=task_id, **options)