Fix wrong type in docstring (#4067)

* Fix wrong type in docstring

* Remove trailing whitespace

celery/canvas.py

@@ -925,9 +925,9 @@ class group(Signature):
         [4, 8]
 
     Arguments:
-        *tasks (Signature): A list of signatures that this group will call.
-            If there's only one argument, and that argument is an iterable,
-            then that'll define the list of signatures instead.
+        *tasks (List[Signature]): A list of signatures that this group will
+            call. If there's only one argument, and that argument is an
+            iterable, then that'll define the list of signatures instead.
         **options (Any): Execution options applied to all tasks
             in the group.