docs: Update api-reference.mdx in beta docs

[morinokami]

Description

• Add import { cx } from "cva"; to the cx code example for consistency.
• Update the imported function in the defineConfig code example.

Additional context

---

What is the purpose of this pull request?

• [ ] Bug fix
• [ ] New Feature
• [X] Documentation update
• [ ] Other

Before submitting the PR, please make sure you do the following

• [X] Read the Contributing Guidelines.
• [X] Follow the Style Guide.
• [X] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [X] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).

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Name	Status	Preview	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	Jan 1, 2024 8:52am
docs-beta	✅ Ready (Inspect)	Visit Preview	Jan 1, 2024 8:52am

[joe-bell]

Thank you!