Fix camelCase typo in expo-router.mdx

[imjared]

If you want to hire me after this brilliant change, please reach out to my secretary.

DESCRIBE YOUR PR

Fixes a capitalization issue in a code snippet. Think of the children!

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• Checked Vercel preview for correctness, including links
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LEGAL BOILERPLATE

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• Sentry Docs contributor guide

[vercel]

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[imjared]

dupe of #11812