core modules docs are misleading

related: https://github.com/import-js/eslint-plugin-import/issues/1390

https://github.com/import-js/eslint-plugin-import?tab=readme-ov-file#importcore-modules

The docs definitely imply that all you need for `import/core-modules` is a list of modules to import. But it should be

```
import/core-modules: [2, electron ]
```

as far as I can tell.

Open the README's import/core-modules section and compare its documented configuration with the syntax shown in the issue and the related issue. Update the example or wording so the required severity and module list are clear, then verify the rendered README reflects the intended configuration.

Written by the indexing model from the issue text.