Doc fixes for SMAPE and R2Score

[SkafteNicki]

What does this PR do?

Fixes #2744

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.

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📚 Documentation preview 📚: https://torchmetrics--2771.org.readthedocs.build/en/2771/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 36%. Comparing base (31ce89e) to head (104cf8c).
Report is 2 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (31ce89e) and HEAD (104cf8c). Click for more details.

HEAD has 388 uploads less than BASE

Flag	BASE (31ce89e)	HEAD (104cf8c)
Windows	17	3
python3.8	18	3
cpu	117	20
torch1.13.1+cpu	18	3
torch2.4.0+cpu	6	1
python3.11	16	3
torch1.12.1+cpu	6	1
Linux	78	13
python3.9	54	9
torch1.11.0+cpu	6	1
torch1.10.2+cpu	6	1
macOS	22	4
python3.10	29	5
torch2.0.1	12	2
torch2.3.1+cpu	12	2
torch2.0.1+cpu	17	3
torch2.2.2+cpu	12	2
torch1.13.1	6	1
torch2.4.0+cu121	6	1
torch2.1.2+cpu	6	1
torch2.4.0	4	1

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2771     +/-   ##
========================================
- Coverage      69%     36%    -33%     
========================================
  Files         329     329             
  Lines       18077   18077             
========================================
- Hits        12499    6579   -5920     
- Misses       5578   11498   +5920