Fix documentation in ERGAS metric

[SkafteNicki]

What does this PR do?

Fixes #2561

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2573.org.readthedocs.build/en/2573/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38%. Comparing base (53fbadf) to head (5797aba).
Report is 99 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2573     +/-   ##
========================================
- Coverage      69%     38%    -31%     
========================================
  Files         314     314             
  Lines       17663   17663             
========================================
- Hits        12181    6691   -5490     
- Misses       5482   10972   +5490