Clarify the term constructor

docs/tutorial.rst

@@ -40,6 +40,7 @@ complicated, a line by line explanation follows below.
 
     dag = DAG('tutorial', default_args=default_args)
 
+    # t1, t2 and t3 are examples of a constructor
     t1 = BashOperator(
         task_id='print_date',
         bash_command='date',