docs(README): update example with using ESM

[kachick]

Follow #909

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Before the change?

> node
Welcome to Node.js v22.10.0.
Type ".help" for more information.
> const { validate } = require("@octokit/graphql-schema");
Uncaught:
Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: No "exports" main defined in

After the change?

• It requires await import for REPL. However, the import statement appears to be intended in this README.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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