Fix Sandboxed reference in AssetService docs

[secret-rare]

AssetService:LoadAssetAsync links to the non-existent Class.Model.Sandboxed Sandboxed is a property of Instance, not Model.

Changes

Change reference to Class.Model.Sandboxed to Class.Instance.Sandboxed

Checks

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[github-actions]

Hi @secret-rare, thanks so much for helping improve the Roblox creator documentation! Our technical writing team will review your pull request soon. In the meantime, please ensure you've read through the README.md, contribution guidelines, and style recommendations.

[aetter]

Hey secret-rare, thanks for the PR. I'm inclined to keep this one as-is since Model inherits from Instance. I get that the property description on Instance is the helpful one, but I feel like it opens a can of worms to link to the base class rather than the class in question, especially in situations where 2-3 inherited classes are involved. Thanks again!