Fix check in mAP

[SkafteNicki]

What does this PR do?

Fixes #1777
Fixes a stupid copy mistake in #1763, that leads to the error nearly always being raised.

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?

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📚 Documentation preview 📚: https://torchmetrics--1784.org.readthedocs.build/en/1784/

[codecov]

Codecov Report

Merging #1784 (1adf2ba) into master (fd90062) will decrease coverage by 47%.
The diff coverage is 0%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1784     +/-   ##
========================================
- Coverage      87%     40%    -47%     
========================================
  Files         254     254             
  Lines       14251   14249      -2     
========================================
- Hits        12456    5761   -6695     
- Misses       1795    8488   +6693