fix(theme-classic): properly highlight code block line numbers

[Josh-Cena]

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

Noticed that the code block number highlighting is not properly applied. This PR fixes it.

Test Plan

Test links

Deploy preview: https://deploy-preview-7341--docusaurus-2.netlify.app/docs/markdown-features/code-blocks/#line-numbering

Related issues/PRs

#7178

[netlify]

✅ [V2]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 62	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 83	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: +9 B (0%)

Total Size: 807 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	51.3 kB	0 B
website/build/assets/css/styles.********.css	105 kB	+9 B (0%)
website/build/assets/js/main.********.js	612 kB	0 B
website/build/index.html	38.8 kB	0 B

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