docs: update Evaluate_RAG_with_LlamaIndex.ipynb

[eltociear]

Summary

minor fix

---

For new content

When contributing new content, read through our contribution guidelines, and mark the following action items as completed:

• [ ] I have added a new entry in registry.yaml (and, optionally, in authors.yaml) so that my content renders on the cookbook website.
• [ ] I have conducted a self-review of my content based on the contribution guidelines:
  • [ ] Relevance: This content is related to building with OpenAI technologies and is useful to others.
  • [ ] Uniqueness: I have searched for related examples in the OpenAI Cookbook, and verified that my content offers new insights or unique information compared to existing documentation.
  • [ ] Spelling and Grammar: I have checked for spelling or grammatical mistakes.
  • [ ] Clarity: I have done a final read-through and verified that my submission is well-organized and easy to understand.
  • [ ] Correctness: The information I include is correct and all of my code executes successfully.
  • [ ] Completeness: I have explained everything fully, including all necessary references and citations.

We will rate each of these areas on a scale from 1 to 4, and will only accept contributions that score 3 or higher on all areas. Refer to our contribution guidelines for more details.