Fix snippet formatting in StudioTheme.yaml

[alex-y-z]

Changes

Removed an extraneous code block that was interfering with an example.

Checks

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[github-actions]

Hi @alex-y-z, 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.