Fixed example code in doc:Custom Agents (#5381)

The Tuple class is never used in CountDownAgent class.

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Co-authored-by: Victor Dibia <victordibia@microsoft.com>

python/packages/autogen-core/docs/src/user-guide/agentchat-user-guide/tutorial/custom-agents.ipynb

@@ -49,7 +49,7 @@
     }
    ],
    "source": [
-    "from typing import AsyncGenerator, List, Sequence, Tuple\n",
+    "from typing import AsyncGenerator, List, Sequence\n",
     "\n",
     "from autogen_agentchat.agents import BaseChatAgent\n",
     "from autogen_agentchat.base import Response\n",
@@ -310,4 +310,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 2
-}
+}