add missing quotes in json samples

[NicolasMassart]

Pull request checklist

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Make sure that:

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Describe the change

Add missing quotes " in JSON code samples in the API page.

Issue fixed

Small fix, no ticket. Was reported on Slack.

Impacted parts

For content changes:

• Documentation content
• Documentation page organization

For tool changes:

• Build and QA tools (e.g., lint)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• GitHub integration

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[rolandtyler]

lgtm