fix(theme-default): fix the capitalization issue of X in socialLinks presetIcons

[wzono]

Summary

Fix the capitalization issue of X in socialLinks presetIcons

Related Issue

No Issue

Checklist

• I have added changeset via pnpm run change.
• I have updated the documentation.
• I have added tests to cover my changes.

[CLAassistant]

All committers have signed the CLA.

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[sanyuan0704]

Thank you