Update agent_vectorstore.ipynb (#2358)

Hi I am learning LangChain and I read that VectorDBQA was changed to
RetrievalQA I thought I could help by making the change if I am wrong
could you give me some feedback I am still learning.

source:
https://blog.langchain.dev/retrieval/#:~:text=Changed%20all%20our,a%20chat%20model

docs/modules/agents/agent_executors/examples/agent_vectorstore.ipynb

@@ -9,7 +9,7 @@
     "\n",
     "This notebook covers how to combine agents and vectorstores. The use case for this is that you've ingested your data into a vectorstore and want to interact with it in an agentic manner.\n",
     "\n",
-    "The reccomended method for doing so is to create a VectorDBQAChain and then use that as a tool in the overall agent. Let's take a look at doing this below. You can do this with multiple different vectordbs, and use the agent as a way to route between them. There are two different ways of doing this - you can either let the agent use the vectorstores as normal tools, or you can set `return_direct=True` to really just use the agent as a router."
+    "The reccomended method for doing so is to create a RetrievalQA and then use that as a tool in the overall agent. Let's take a look at doing this below. You can do this with multiple different vectordbs, and use the agent as a way to route between them. There are two different ways of doing this - you can either let the agent use the vectorstores as normal tools, or you can set `return_direct=True` to really just use the agent as a router."
    ]
   },
   {