gh-141617: clarify concurrent.futures.ThreadPoolExecutor deadlock example

[Yashp002]

This PR fixes an incomplete code example in the concurrent.futures documentation.

The second example in the deadlock section was missing a .result() call on the executor.submit() invocation, which is necessary to demonstrate the deadlock behavior described in the comment.

Changes:

• Added .result() call to complete the example in Doc/library/concurrent.futures.rst

Fixes #141617

• Issue: Incomplete example in concurrent.future documentation #141617

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📚 Documentation preview 📚: https://cpython-previews--141620.org.readthedocs.build/

[picnixz]

The other place where to add the comment was after b = executor.submit(wait_on_a) just a few lines above not in the documentation for submit.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[Yashp002]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@picnixz: please review the changes made to this pull request.

[Yashp002]

Hi @picnixz Just checking in on this PR. I'm happy to address any remaining feedback or make changes as needed. Please let me know if there's anything I should update. Thanks.

[picnixz]

Keep the blank lines where they are.