fix(ex04): typo (#195)

* fix: renmae `roll()` to `world()`

DESC: According to the functionality, the function is built by
extending `ex01.py`, thus it's more natural to use the function
`world()`

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