Docs: Fix Modal with Dropdown composition

[literat]

Description

The Dropdown was using an old structure with the render prop.

Additional context

It is not working here: https://spirit-design-system-storybook.netlify.app/?path=/story/examples-compositions-modals--modal-with-dropdown

Issue reference

[netlify]

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[coveralls]

coverage: 78.531%. remained the same
when pulling 5ae54b6 on docs/fix-modal-dropdown-composition
into 4c88e55 on integration/release-v2.