fix: correct video tag

[001123]

Fix incorrect video tag.
Link: https://learn.microsoft.com/en-us/adaptive-cards/

Microsoft Reviewers: Open in CodeFlow

[learn-build-service-prod]

Learn Build status updates of commit 4011153:

💡 Validation status: suggestions

File	Status	Preview URL	Details
AdaptiveCards/index.md	💡Suggestion		Details

AdaptiveCards/index.md

• Line 2, Column 1: [Suggestion: description-missing - See documentation] Missing required attribute: 'description'.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel