Update babel-jest example

[nicolo-ribaudo]

Ref: jestjs/jest#11444

@SimenB Btw, babel-jest could add export { createTransformer } (other than the default export) to avoid the need of .default when used from CommonJS or native ESM.

[SimenB]

Yep, I do think we'll re-add it as it's sorta wonky

[SimenB]

You can also provide this in Jest's config, that might be better than using createTransformer directly?

{
  "transform": [".*js", {"rootMode": "upward"}]
}

(don't copy this snippet without verifying, written from memory)

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