docs: fix broken BaseOperator link in templates-ref (#60477)

[Sanjays2402]

Closes #60477

Fixes broken BaseOperator link in templates-ref

The template variable reference table in airflow-core/docs/templates-ref.rst
linked the {{ task }} variable to
:class:~airflow.models.baseoperator.BaseOperator``. That module no longer
exists on main — `BaseOperator` now lives in the Task SDK — so the link is
dead and renders as an unresolvable reference.

This updates the reference to the canonical public path
:class:~airflow.sdk.BaseOperator``, which is the form already used throughout
the rest of the core docs (e.g. howto/custom-operator.rst,
`authoring-and-scheduling/deferring.rst`, `tutorial/fundamentals.rst`).

Doc-only change — no behavior change, no newsfragment required.

---

^ Add meaningful description above

• Description above provides context of the change
• Unit tests coverage for changes are not needed (docs-only change)
• Target Github ISSUE in description if exists
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as reviewers and add my changes to a public repo

[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 Contributors' Guide
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example Dag that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack