Replace undefined variable name in documentation code snippet

[leykun10]

Corrected the variable name from 'output' to 'result' in the code snippet in running_agents.md to match the actual code usage.
the variable output was not defined and it is implying the variable result from the first run.

Previous:

async def main():
    agent = Agent(name="Assistant", instructions="Reply very concisely.")
    with trace(workflow_name="Conversation", group_id=thread_id):
        # First turn
        result = await Runner.run(agent, "What city is the Golden Gate Bridge in?")
        print(result.final_output)
        # San Francisco

        # Second turn
        new_input = output.to_input_list() + [{"role": "user", "content": "What state is it in?"}]
        result = await Runner.run(agent, new_input)
        print(result.final_output)
        # California

Updated:

async def main():
  agent = Agent(name="Assistant", instructions="Reply very concisely.")

  with trace(workflow_name="Conversation", group_id=thread_id):
      # First turn
      result = await Runner.run(agent, "What city is the Golden Gate Bridge in?")
      print(result.final_output)
      # San Francisco

      # Second turn
      new_input = result.to_input_list() + [{"role": "user", "content": "What state is it in?"}]
      result = await Runner.run(agent, new_input)
      print(result.final_output)
      # California```