Update setup.md with LlamaIndex updated field. (#363)

In setup.md in the LlamaIndex setup section - Changed GPTSimpleVectorIndex to GPTVectorStoreIndex to match the updated field in LlamaIndex. 

Fixes issue #307.

docs/providers/llama/setup.md

@@ -33,7 +33,7 @@ with ChatGPT and your external data.
 
 
 **Different Index Types**
-By default, we use a `GPTSimpleVectorIndex` to store document chunks in memory, 
+By default, we use a `GPTVectorStoreIndex` to store document chunks in memory, 
 and retrieve top-k nodes by embedding similarity.
 Different index types are optimized for different data and query use-cases.
 See this guide on [How Each Index Works](https://gpt-index.readthedocs.io/en/latest/guides/primer/index_guide.html) to learn more.
@@ -48,4 +48,4 @@ You can launch the test suite with this command:
 
 ```bash
 pytest ./tests/datastore/providers/llama/test_llama_datastore.py
-```
+```