fix: Update model name in hello_world agent

Co-authored-by: Xuan Yang <xygoogle@google.com>
PiperOrigin-RevId: 912743254

Author: xuanyang15

contributing/samples/hello_world/agent.py

@@ -65,7 +65,7 @@ async def check_prime(nums: list[int]) -> str:
 
 
 root_agent = Agent(
-    model='projects/adk-cat/locations/us-central1/publishers/google/models/gemini-2.5-flash',
+    model='gemini-2.5-flash',
     name='hello_world_agent',
     description=(
         'hello world agent that can roll a dice of 8 sides and check prime'