[css-layout-api] const childFragments redefined in example 10

[fred-wang]

See https://drafts.css-houdini.org/css-layout-api-1/#example-c28dfd2e

    const childFragments = [];
    const childConstraints = { availableInlineSize, availableBlockSize };

    const childFragments = await Promise.all(children.map((child) => {
        return child.layoutNextFragment(childConstraints);
    }));

This is causing a JS error.