fix(chat): add assistantId to step 3 of quickstart

Author: goosewin

fern/chat/chat.mdx

@@ -69,6 +69,7 @@ Key features:
       -H "Authorization: Bearer YOUR_API_KEY" \
       -H "Content-Type: application/json" \
       -d '{
+        "assistantId": "your-assistant-id",
         "previousChatId": "chat_abc123",
         "input": "Tell me about your features"
       }'