Update unit2/langgraph/mail_sorting.ipynb

With the original import line:
from langfuse.callback import CallbackHandler

an error is raised:
from langfuse.integrations.langchain import LangfuseCallbackHandler
ModuleNotFoundError: No module named 'langfuse.integrations'

After researching the issue, I found that in recent versions of Langfuse (including v3.x), the correct import for the LangChain callback handler is:
from langfuse.langchain import CallbackHandler

unit2/langgraph/mail_sorting.ipynb

@@ -361,7 +361,7 @@
    "outputs": [],
    "execution_count": null,
    "source": [
-    "from langfuse.callback import CallbackHandler\n",
+    "from langfuse.langchain import CallbackHandler\n",
     "\n",
     "# Initialize Langfuse CallbackHandler for LangGraph/Langchain (tracing)\n",
     "langfuse_handler = CallbackHandler()"