Fixes ref calling -> guide_calling

docs/userguide/canvas.rst

@@ -16,7 +16,7 @@ Subtasks
 .. versionadded:: 2.0
 
 You just learned how to call a task using the tasks ``delay`` method
-in the :ref:`calling <calling>` guide, and this is often all you need,
+in the :ref:`calling <guide-calling>` guide, and this is often all you need,
 but sometimes you may want to pass the signature of a task invocation to
 another process or as an argument to another function, for this Celery uses
 something called *subtasks*.