Fixed triple quotes in task group example

[mati-o]

Just noticed that the code sample for task groups wasn't rendered well because the triple quotes did not close properly, so I have added the two missing " chars.

[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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[josh-fell]

Nice catch!

[josh-fell]

Nice catch!

[boring-cyborg]

Awesome work, congrats on your first merged pull request!