Docs: multilabel is also a string

[adamjstewart]

What does this PR do?

Task can be 'binary', 'multiclass', or 'multilabel', but for some reason multilabel is never written with string quotes unlike the other two. This PR updates multilabel for consistency.

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--2996.org.readthedocs.build/en/2996/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40%. Comparing base (34fca16) to head (fa7a44d).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (34fca16) and HEAD (fa7a44d). Click for more details.

HEAD has 168 uploads less than BASE

Flag	BASE (34fca16)	HEAD (fa7a44d)
torch2.0.1+cpu	11	3
python3.10	35	9
Windows	8	2
cpu	55	14
macOS	12	3
python3.12	12	3
torch2.6.0	4	1
torch2.0.1	8	2
torch2.6.0+cpu	4	1
Linux	35	9
torch2.4.1+cpu	8	2
python3.11	4	1
torch2.1.2+cpu	4	1
torch2.3.1+cpu	4	1
python3.9	4	1
torch2.5.0+cpu	8	2
torch2.2.2+cpu	4	1
gpu	2	0
unittest	2	0

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2996     +/-   ##
========================================
- Coverage      70%     40%    -29%     
========================================
  Files         352     338     -14     
  Lines       19615   19436    -179     
========================================
- Hits        13658    7834   -5824     
- Misses       5957   11602   +5645     

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.