Missing UserMessage import (#6583)

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## Why are these changes needed?

The code block fails to execute without the import

## Related issue number

N/A

## Checks

- [x ] I've included any doc changes needed for
<https://microsoft.github.io/autogen/>. See
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Co-authored-by: Victor Dibia <victordibia@microsoft.com>

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

@@ -165,6 +165,7 @@
     "from autogen_ext.auth.azure import AzureTokenProvider\n",
     "from autogen_ext.models.openai import AzureOpenAIChatCompletionClient\n",
     "from azure.identity import DefaultAzureCredential\n",
+    "from autogen_core.models import UserMessage\n",
     "\n",
     "# Create the token provider\n",
     "token_provider = AzureTokenProvider(\n",