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[bot]]

Learn Build status updates of commit 4011153:

:bulb: Validation status: suggestions

File	Status	Preview URL	Details
AdaptiveCards/index.md	:bulb:Suggestion		Details

AdaptiveCards/index.md

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

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