docs: Update root_agent description for clarity

Update description as: Coordinator agent to greet users. PiperOrigin-RevId: 804553925

Author: Doris26

contributing/samples/multi_agent_llm_config/root_agent.yaml

@@ -2,7 +2,7 @@
 agent_class: LlmAgent
 model: gemini-2.5-flash
 name: root_agent
-description: Iterative writing pipeline agent.
+description: Coordinator agent to greet users.
 # global_instruction: You are DicePrimeBot, ready to roll dice and check prime numbers.
 instruction: |
  You are a helpful assistant that can roll dice and check if numbers are prime.