Fix get_context_data doctest import

[orige]

This commit fixes the doctest replacing the old import - from
airflow.task.context - to the newer one.

[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/master/CONTRIBUTING.rst)
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[boring-cyborg]

Awesome work, congrats on your first merged pull request!