docs: remove duplicated heading section in rag.ipynb (#5674)

Co-authored-by: Brace Sproul <[email protected]>

docs/core_docs/docs/tutorials/rag.ipynb

@@ -73,16 +73,6 @@
         "export LANGCHAIN_API_KEY=\"...\"\n",
         "```\n",
         "\n",
-        "## Preview\n",
-        "\n",
-        "In this guide we’ll build a QA app over as website. The specific website we will use isthe [LLM Powered Autonomous\n",
-        "Agents](https://lilianweng.github.io/posts/2023-06-23-agent/) blog post\n",
-        "by Lilian Weng, which allows us to ask questions about the contents of\n",
-        "the post.\n",
-        "\n",
-        "We can create a simple indexing pipeline and RAG chain to do this in ~20\n",
-        "lines of code:\n",
-        "\n",
         "```{=mdx}\n",
         "import ChatModelTabs from \"@theme/ChatModelTabs\";\n",
         "\n",