docs(start/concepts): fix warning

[kno-raziel]

The is being treated as markup by the mdx, and is being rendered as a tag on the page resulting in :

<docs-warning>
Don't forget to add an `
<outlet>` 
to your layout where you would like child route elements to be rendered. Using `children` will not work as expected.
</outlet>
</docs-warning>

instead of

<docs-warning>
Don't forget to add an <code>`<outlet>`</code> to your layout where you would like child route elements to be rendered. Using <code>`{children}`</code> will not work as expected.
</docs-warning>

[changeset-bot]

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[remix-cla-bot]

Hi @kno-raziel,

Welcome, and thank you for contributing to React Router!

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[timdorr]

@kno-raziel Can you sign the CLA so I can merge this? Thanks!

[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[timdorr]

Thanks!