chore: correct import in readme (#154)

Author: twishabansal

packages/toolbox-core/README.md

@@ -190,6 +190,7 @@ from toolbox_core import ToolboxClient
 from langchain_google_vertexai import ChatVertexAI
 from langgraph.graph import StateGraph, MessagesState, START, END
 from langgraph.prebuilt import ToolNode
+from langchain.tools import StructuredTool
 
 toolbox = ToolboxClient("http://127.0.0.1:5000")
 tools = await toolbox.load_toolset()
@@ -417,4 +418,4 @@ This project is licensed under the Apache License 2.0. See the
 
 # Support
 
-If you encounter issues or have questions, check the existing [GitHub Issues](https://github.com/googleapis/genai-toolbox/issues) for the main Toolbox project.
+If you encounter issues or have questions, check the existing [GitHub Issues](https://github.com/googleapis/genai-toolbox/issues) for the main Toolbox project.