Fix misalignment in sample codes for data mutation guide

[mahito1594]

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Description(required)

Fix highlighting misalignment in samples codes.

This PR addresses syntax highlighting issues that were causing misalignment in sample code blocks. The specific fixes are applied to:

• The useSubmission reference documentation in Solid Router
• The "Data mutation" guide in Solid Start

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• Suggested label(s) (optional): documentation

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[LadyBluenotes]

Thank you for catching this!