chore: Update agent.py

Merge #1136

COPYBARA_INTEGRATE_REVIEW=#1136 from google:hangfei-patch-1 63ff81e
PiperOrigin-RevId: 766913586

Author: hangfei

contributing/samples/hello_world/agent.py

@@ -68,7 +68,7 @@ async def check_prime(nums: list[int]) -> str:
 
 root_agent = Agent(
     model='gemini-2.0-flash',
-    name='data_processing_agent',
+    name='hello_world_agent',
     description=(
         'hello world agent that can roll a dice of 8 sides and check prime'
         ' numbers.'