Grammar and punctuation fixes in AI Experiments plugin docs

[mehrazmorshed]

This pull request improves grammar, punctuation, and clarity in the AI Experiments plugin documentation.

Changes include:

• Replacing a comma splice with a semicolon.
• Adding the missing article in the Slack page reference.
• Ensuring punctuation consistency.
• Correcting pronoun agreement when referring to the WordPress.org support forums.

No functional changes were made; this update is limited to documentation improvements.

[github-actions]

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Co-authored-by: mehrazmorshed <mehrazmorshed@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.89%. Comparing base (6478545) to head (3ca3e23).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #164   +/-   ##
==========================================
  Coverage      46.89%   46.89%           
  Complexity       208      208           
==========================================
  Files             19       19           
  Lines           1271     1271           
==========================================
  Hits             596      596           
  Misses           675      675           

Flag	Coverage Δ
unit	46.89% <ø> (ø)

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