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?

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📚 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).

❌ Your project check has failed because the head coverage (40%) is below the target coverage (95%). You can increase the head coverage or adjust the target coverage.

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

HEAD has 112 uploads less than BASE

Flag	BASE (34fca16)	HEAD (fa7a44d)
torch2.0.1+cpu	8	3
python3.10	26	9
Windows	6	2
cpu	41	14
macOS	9	3
python3.12	9	3
torch2.6.0	3	1
torch2.0.1	6	2
torch2.6.0+cpu	3	1
Linux	26	9
torch2.4.1+cpu	6	2
python3.11	3	1
torch2.1.2+cpu	3	1
torch2.3.1+cpu	3	1
python3.9	3	1
torch2.5.0+cpu	6	2
torch2.2.2+cpu	3	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     

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