docs: fix announcement bar css selector

[3v0k4]

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

#10945 changed the class for the announcement bar, which broke the current CSS selector.

Before http://docusaurus.io/:

After https://deploy-preview-11262--docusaurus-2.netlify.app/:

Test Plan

Visit the links above to test the change.

Test links

https://deploy-preview-11262--docusaurus-2.netlify.app/

Related issues/PRs

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🟠 69	🟢 98	🟢 100	🟢 100	Report
/docs/installation	🟠 50	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 71	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 59	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🔴 45	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 60	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 71	🟢 100	🟢 100	🟠 86	Report

[slorber]

Thanks, wanted to fix it 😄