Update 'first RAG agent' in gettingstarted doc (#867)

# What does this PR do?

Fix documentation to reflect new API


## Test Plan
Before:

User> What are the top 5 topics that were explained? Only list succinct
bullet points.
inference> I'm ready to help, but we haven't discussed any topics yet!
This is the start of our conversation. What would you like to talk
about? I can summarize our discussion at the end if you'd like.


Run with the change, observe relevant response

<img width="1029" alt="image"
src="https://github.com/user-attachments/assets/a7dece3c-e8b4-4a60-9092-ba544c87dffd"
/>



## Sources

Please link relevant resources if necessary.


## Before submitting

- [x] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

Co-authored-by: Eric Huang (AI Platform) <[email protected]>

docs/source/getting_started/index.md

@@ -153,7 +153,7 @@ agent_config = AgentConfig(
     # Define tools available to the agent
     toolgroups = [
         {
-          "name": "builtin::memory",
+          "name": "builtin::rag",
           "args" : {
             "vector_db_ids": [vector_db_id],
           }