chore: credit the Lotus paper in the sem ops notebook (#1218)

Author: sycai

notebooks/experimental/semantic_operators.ipynb

@@ -32,6 +32,8 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "We implemented the semantics operators based on the idea in the \"Lotus\" paper: https://arxiv.org/pdf/2407.11418.\n",
+    "\n",
     "This notebook gives you a hands-on preview of semantic operator APIs powered by LLM. The demonstration is devided into two sections: \n",
     "\n",
     "The first section introduces the API syntax with some simple examples. We aim to get you familiar with how BigFrames semantic operators work. \n",