Update ex04.py

怀疑是一个typo。
从函数的功能来看应该是对 `ex01.py ` 的扩展，沿用 `world()` 函数名更自然。

example/ex04_reply/ex04.py

@@ -14,7 +14,7 @@
 # register command
 # invoke this via saying `!hello` in channel
 @bot.command(name='hello')
-async def roll(msg: Message):
+async def world(msg: Message):
     # quote reply
     await msg.reply('world for you!')
     # msg from reply_temp & send_temp will be cleaned when you restart client or refresh browser