[Constrained Beam Search] Adding Notebook Example & Minor Typo Fix
# [CBS] Adding Notebook Example & Minor Typo Fix

Very lightweight PR for:
- a fix to a somewhat embarrassing copy-and-paste error I made in my initial PR
- Thought it'd be nice if the notebook on `huggingface/blog` could be added to the list of Pytorch notebooks as others have done (both the Colab and SageMaker link imports from the `huggingface/blog` `master` branch with no issue.


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@patrickvonplaten 



_The documentation is not available anymore as the PR was closed or merged._<|||||>The current test fail is unrelated to the PR (cbs is separated from `sample_generate`):

```
-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=========================== short test summary info ============================
FAILED tests/pegasus/test_modeling_pegasus.py::PegasusStandaloneDecoderModelTest::test_sample_generate
=== 1 failed, 5253 passed, 4576 skipped, 4839 warnings in 498.21s (0:08:18) ====
```