Fix Perplexity docs

[AtomicVar]

What does this PR do?

Fixes the docs of Perplexity: it has incorrect indentation so it's not rendered right.

Before the fix:

After the fix:

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

No. Building the docs is hard for me. I cannot install the lai-sphinx-theme package easily. I refered to pytorch-lightning's Makefile to figure out that it can be installed using AWS S3 CLI.

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📚 Documentation preview 📚: https://torchmetrics--2391.org.readthedocs.build/en/2391/

[codecov]

Codecov Report

Merging #2391 (b72b0b8) into master (4c999b8) will decrease coverage by 29%.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2391     +/-   ##
========================================
- Coverage      69%     40%    -29%     
========================================
  Files         307     307             
  Lines       17352   17352             
========================================
- Hits        11961    6923   -5038     
- Misses       5391   10429   +5038