docs: correct and stabilize dehumanize month examples

[Likio3000]

Description of Changes

The guide and dehumanize() docstring show April 20 plus “in a month” becoming May 18. The implementation applies a calendar-month shift, so the shown result should be May 20; the guide's Hindi example has the same incorrect result.

Correct all three month results and construct the same fixed April 20 timestamp in these examples so the displayed inputs and outputs are reproducible. The two-days-ago examples remain April 18. This changes documentation only, including the docstring; it leaves the parameter-name correction in #1355 untouched.

Validation: followed English/Hindi locale matching through dehumanize() to shift(months=1), read the existing shift assertions, and confirmed by AST comparison that all executable Python is identical after excluding the changed docstring. Source bytes and the final diff were checked. No Arrow/dateutil imports, dependency installs, project tests or documentation build were run locally. This is a draft pending those checks.

AI assistance: prepared with OpenAI Codex, with source and diff review.

Pull Request Checklist

• 🧪 Added tests for changed code. (No executable-code change.)
• 🛠️ All tests pass when run locally. (Not run.)
• 🧹 All linting checks pass when run locally. (Not run.)
• 📚 Updated documentation.
• ⏩ Based on master at 2224255c4acc594d734cef0bbc83360452a67983.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (6cc88d4).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
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  Hits          2315      2315           

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