Fixed typo where guide was attempting to define a class object using the 'def' keyword.

docs/userguide/tasks.rst

@@ -734,7 +734,7 @@ will do roughly this behind the scenes:
 .. code-block:: python
 
     @task
-    def AddTask(Task):
+    class AddTask(Task):
 
         def run(self, x, y):
             return x + y