docs: add missing InputGuardrailTripwireTriggered import to quickstart example

Fixes NameError when handling guardrail exceptions by including the required import in the docs.

Author: slubbers-openai

docs/quickstart.md

@@ -95,8 +95,10 @@ You can define custom guardrails to run on the input or output.
 
 ```python
 from agents import GuardrailFunctionOutput, Agent, Runner
+from agents.exceptions import InputGuardrailTripwireTriggered
 from pydantic import BaseModel
 
+
 class HomeworkOutput(BaseModel):
     is_homework: bool
     reasoning: str
@@ -122,6 +124,7 @@ Let's put it all together and run the entire workflow, using handoffs and the in
 
 ```python
 from agents import Agent, InputGuardrail, GuardrailFunctionOutput, Runner
+from agents.exceptions import InputGuardrailTripwireTriggered
 from pydantic import BaseModel
 import asyncio