fix: use RAG term instead of generative AI

[akocbek]

Description

Issue: https://github.com/terraform-ibm-modules/stack-retrieval-augmented-generation/issues/48

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[vburckhardt]

Hi @akocbek - I had made some the changes at https://github.com/terraform-ibm-modules/stack-retrieval-augmented-generation/tree/dev - I think we can merge with what you have. Could you open a PR against the dev branch?