title: Callback for join command is missing "ctx" parameter
body: <p>I'm trying to make a music cog for my bot. I'm currently making commands for the bot to join and leave a voice channel. But everytime i try to launch both of these commands, the same error occurs.</p>
  
  <pre><code>Ignoring exception in on_message
  Traceback (most recent call last):
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\core.py", line 663, in _parse_arguments
      next(iterator)
  StopIteration
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\client.py", line 312, in _run_event
      await coro(*args, **kwargs)
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\bot.py", line 930, in on_message
      await self.process_commands(message)
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\bot.py", line 927, in process_commands
      await self.invoke(ctx)
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\bot.py", line 892, in invoke
      await ctx.command.invoke(ctx)
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\core.py", line 790, in invoke
      await self.prepare(ctx)
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\core.py", line 751, in prepare
      await self._parse_arguments(ctx)
    File "C:\Users\Daniel\AppData\Local\Programs\Python\Python38\lib\site-packages\discord\ext\commands\core.py", line 666, in _parse_arguments
      raise discord.ClientException(fmt.format(self))
  discord.errors.ClientException: Callback for join/leave command is missing "ctx" parameter.
  </code></pre>
  
  <p>Here's the code i have:</p>
  
  <pre><code>import discord
  from discord.ext import commands
  
  import youtube_dl
  
  class Music(commands.Cog):
  
      def __init__(self, client):
          self.client = client
  
      @commands.command()
      async def join(ctx):
          channel = ctx.message.author.voice.channel
          await channel.connect()
  
      @commands.command()
      async def leave(ctx):
          await ctx.voice_client.diconnect()
  
  
  def setup(client):
      client.add_cog(Music(client))
  </code></pre>
  
  <p>Anyone know how i can fix this ?</p>