Minor fixes on /lib

[PHBourquin]

Also with a minor README.md update that isn't intuitive

Checklist

• [X ] I've read the Contributing Guidelines and the Code of Conduct
• [X ] I've completed the Contributor License Agreement

Pull Request Details

Following https://github.com/facebook/facebook-business-sdk-codegen/issues/79, this fixes 2 minor issues in the lib/ files along + a small READM.md update. Indeed, it wasn't implicit as an external contributor that I had to checkout the SDKs and update version_path.json base path towards them to be able to generate the clients, and this bit of information would have been welcome :)

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