Update calling.rst

Fixed typo variable on example code of Connections.
Conflicts:
	docs/userguide/calling.rst

docs/userguide/calling.rst

@@ -447,7 +447,7 @@ 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(n) for n in numbers).apply_async()
+    >>> res = group(add.s(i) for i in numbers).apply_async()
 
     >>> res.get()
     [4, 8, 16, 32]