Fix docs on creating CustomOperator (#8678)

(cherry picked from commit 5e1c33a)

docs/howto/custom-operator.rst

@@ -148,7 +148,7 @@ the operator.
                 self.name = name
 
             def execute(self, context):
-                message = "Hello from {}".format(name)
+                message = "Hello from {}".format(self.name)
                 print(message)
                 return message
 
@@ -157,9 +157,9 @@ You can use the template as follows:
 .. code:: python
 
         with dag:
-            hello_task = HelloOperator(task_id='task_id_1', dag=dag, name='{{ task_id }}')
+            hello_task = HelloOperator(task_id='task_id_1', dag=dag, name='{{ task_instance.task_id }}')
 
-In this example, Jinja looks for the ``name`` parameter and substitutes ``{{ task_id }}`` with
+In this example, Jinja looks for the ``name`` parameter and substitutes ``{{ task_instance.task_id }}`` with
 ``task_id_1``.