docs(plugin-legacy): outdated csp hash (fix #12112)

[sun0day]

Description

fix #12112

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[sun0day]

Maybe we should just explain how to retrieve the latest csp hash in case forgetting to update the README.md.

Run `node --input-type=module -e "import {cspHashes} from '@vitejs/plugin-legacy'; console.log(cspHashes.map(h => 'sha256-'+h))"` to retrieve the value.

[bluwy]

I think it's probably fine for now to update this manually as they don't change often 👍