Add missed import in the Trigger Rules example

[pigidser]

An example of the Trigger Rules chapter raises the error while loading to UI:

Broken DAG: [/opt/airflow/dags/branch_without_trigger.py]
Traceback (most recent call last):
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/opt/airflow/dags/branch_without_trigger.py", line 16, in <module>
    @task.branch(task_id="branching")
NameError: name 'task' is not defined

It is needed to add the next import:

from airflow.decorators import task

---

^ Add meaningful description above

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[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/main/CONTRIBUTING.rst)
Here are some useful points:

• Pay attention to the quality of your code (flake8, mypy and type annotations). Our pre-commits will help you with that.
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[boring-cyborg]

Awesome work, congrats on your first merged pull request!