ai21: AI21 Jamba docs (#21978)

- Updated docs to have an example to use Jamba instead of J2

---------

Co-authored-by: Asaf Gardin <asafg@ai21.com>
Co-authored-by: Erick Friis <erick@langchain.dev>

docs/docs/integrations/chat/ai21.ipynb

@@ -95,7 +95,7 @@
     "from langchain_ai21 import ChatAI21\n",
     "from langchain_core.prompts import ChatPromptTemplate\n",
     "\n",
-    "chat = ChatAI21(model=\"j2-ultra\")\n",
+    "chat = ChatAI21(model=\"jamba-instruct\")\n",
     "\n",
     "prompt = ChatPromptTemplate.from_messages(\n",
     "    [\n",
@@ -107,14 +107,6 @@
     "chain = prompt | chat\n",
     "chain.invoke({\"english_text\": \"Hello, how are you?\"})"
    ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "id": "c159a79f",
-   "metadata": {},
-   "outputs": [],
-   "source": []
   }
  ],
  "metadata": {