Fix single update in pearson corrcoef

[SkafteNicki]

What does this PR do?

Fixes #2014

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?

Make sure you had fun coding 🙃

---

📚 Documentation preview 📚: https://torchmetrics--2019.org.readthedocs.build/en/2019/

[codecov]

Codecov Report

Merging #2019 (d35914a) into master (f6760b5) will decrease coverage by 48%.
Report is 1 commits behind head on master.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2019     +/-   ##
========================================
- Coverage      87%     39%    -48%     
========================================
  Files         270     270             
  Lines       15574   15572      -2     
========================================
- Hits        13477    6059   -7418     
- Misses       2097    9513   +7416