Fix the custom test result timestamp in milliseconds

[mtseafood]

Describe your changes:

This pull request updates the "Test Result" section in the Data Quality Observability Custom Tests documentation. The changes include:

• Clarifying the JSON payload timestamp format to create and run custom test result.

For more details, refer to the discussion on Slack: Slack Link

What changes did you make?

• Added a comment explaining the Unix timestamp format for writing TestCaseResults.

Why did you make them?

• The Unix timestamp in milliseconds is the correct format that the API accepts, ensuring accurate and consistent test results.

Type of change:

• Bug fix
• Improvement
• New feature
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation

Checklist:

• I have read the CONTRIBUTING document.
• My PR title is Fixes <issue-number>: <short explanation>
• I have commented on my code, particularly in hard-to-understand areas.
• For JSON Schema changes: I updated the migration scripts or explained why it is not needed.

[github-actions]

Hi there 👋 Thanks for your contribution!

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