docs: fix package name in babel config docs

[kyletsang]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

The documentation for customizing the babel config says to install @docusaurus/theme-live-codeblock, then use the @docusaurus/babel/preset in the config file. This should tell the user to install @docusaurus/babel instead.

Test Plan

N/A

Test links

Deploy preview: https://deploy-preview-10666--docusaurus-2.netlify.app/

Related issues/PRs

N/A

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[github-actions]

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[slorber]

thanks 👍