fix(docs): Update rag.mdx 'llm' variable to 'model' (#1314)

The guide uses 'model' as variable name for model instance. An example
wrongly uses 'llm' further down the guide.

## Overview
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Author: bzj55pnkxy-droid

src/oss/langchain/rag.mdx

@@ -842,7 +842,7 @@ class RetrieveDocumentsMiddleware(AgentMiddleware[State]):
 
 
 agent = create_agent(
-    llm,
+    model,
     tools=[],
     middleware=[RetrieveDocumentsMiddleware()],
 )