Fixes typo in calling guide. Closes #2479

docs/userguide/calling.rst

@@ -449,7 +449,7 @@ Though this particular example is much better expressed as a group:
     >>> from celery import group
 
     >>> numbers = [(2, 2), (4, 4), (8, 8), (16, 16)]
-    >>> res = group(add.s(i) for i in numbers).apply_async()
+    >>> res = group(add.s(i, j) for i, j in numbers).apply_async()
 
     >>> res.get()
     [4, 8, 16, 32]