Update quickstart.mdx

[rocky-terracotta]

Fix broken tutorial code - message in second addMessage() should be wrapped double quotes

// Example usage
await addMessage(
  'user',
  'Hello, can you help me with a task?'
);
await addMessage(
  'assistant',
  'Of course! I'd be happy to help.'
);

From https://docs.turso.tech/agentfs/quickstart#create-your-first-agent