Update chatbot.ipynb (langchain-ai#27243)

Async invocation:
remove : from at the end of line 
line 441 because there is not any structure block after it.

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docs/docs/tutorials/chatbot.ipynb

@@ -438,7 +438,7 @@
     "app = workflow.compile(checkpointer=MemorySaver())\n",
     "\n",
     "# Async invocation:\n",
-    "output = await app.ainvoke({\"messages\": input_messages}, config):\n",
+    "output = await app.ainvoke({\"messages\": input_messages}, config)\n",
     "output[\"messages\"][-1].pretty_print()\n",
     "```\n",
     "\n",