fix: Using nltk version 3.8.1 Azure Databricks Notebook longchain.ipynb to fix failing test

[FarrukhMasud]

With release of new version of NLTK (3.9.1), the unit test started failing.

What changes are proposed in this pull request?

The version of NLTK is pinned to 3.8.1 in the notebook

How is this patch tested?

I have manually ran the Longchain.ipynb and validated that it is not failing anymore.

Does this PR change any dependencies?

• No. You can skip this section.
• Yes. Make sure the dependencies are resolved correctly, and list changes here.

Does this PR add a new feature? If so, have you added samples on website?

• No. You can skip this section.
• Yes. Make sure you have added samples following below steps.

1. Find the corresponding markdown file for your new feature in website/docs/documentation folder.
   Make sure you choose the correct class estimators/transformers and namespace.
2. Follow the pattern in markdown file and add another section for your new API, including pyspark, scala (and .NET potentially) samples.
3. Make sure the DocTable points to correct API link.
4. Navigate to website folder, and run yarn run start to make sure the website renders correctly.
5. Don't forget to add <!--pytest-codeblocks:cont--> before each python code blocks to enable auto-tests for python samples.
6. Make sure the WebsiteSamplesTests job pass in the pipeline.

[FarrukhMasud]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.51%. Comparing base (feed209) to head (24a0af7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2309      +/-   ##
==========================================
- Coverage   84.57%   84.51%   -0.07%     
==========================================
  Files         326      326              
  Lines       16769    16769              
  Branches     1480     1480              
==========================================
- Hits        14183    14172      -11     
- Misses       2586     2597      +11     

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