Fix math typo in confusion matrix doc

[sweber1]

What does this PR do?

Fix a doc typo for MulticlassConfusionMatrix, specifically in the equation that describes false positive calculation from the confusion matrix.
The sum in the equation is over i (the rows), therefore the result corresponds to the number of true negatives for class j, not i.

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

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📚 Documentation preview 📚: https://torchmetrics--2634.org.readthedocs.build/en/2634/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41%. Comparing base (98e7130) to head (19df354).
Report is 100 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2634     +/-   ##
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- Coverage      69%     41%    -28%     
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  Files         314     314             
  Lines       17716   17716             
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- Hits        12208    7271   -4937     
- Misses       5508   10445   +4937