Use DAG context in dag examples

The current code of dag examples is in the form of:
```
dag = DAG(whatever)

task_1 = Operator(task_id='dfsa', 
                               dag=dag)
```

As it is said in the docs (or in some article written by a commiter somewhere) it is better to have:
`with DAG(whatever):
`
I don't have time right now to make the changes