[yaml] fix documentation and tests for flags

[derrickaw]

Fixes #37980 and some minor typos.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.88%. Comparing base (76f6e6d) to head (74b78c7).
⚠️ Report is 196 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40617       +/-   ##
=============================================
+ Coverage     56.16%   79.88%   +23.71%     
=============================================
  Files          1124      488      -636     
  Lines        178336    82432    -95904     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65852    -34318     
+ Misses        75645    16580    -59065     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.88% <ø> (+0.33%)	⬆️

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[derrickaw]

/gemini review

[github-actions]

📋 Review Summary

This PR updates the Beam YAML testing documentation to explicitly note that both --tests and --test CLI flags are supported, fixes minor typos in the documentation, and expands unit test coverage in main_test.py to verify argument parsing and test execution for both flag variants.

🔍 General Feedback

• The unit test additions in sdks/python/apache_beam/yaml/main_test.py cleanly validate both --tests and --test flags via _parse_arguments and run_tests.
• Documentation updates and typo fixes in website/www/site/content/en/documentation/sdks/yaml-testing.md are clear and accurate.

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[github-actions]

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R: @chamikaramj for label yaml.
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