Update README to have a valid example.

README.md

@@ -46,10 +46,9 @@ async def on_ready():
 @client.event
 async def on_message(message):
     if message.content.startswith('!test'):
-        logs = await client.logs_from(message.channel, limit=100)
         counter = 0
         tmp = await client.send_message(message.channel, 'Calculating messages...')
-        for log in logs:
+        async for log in client.logs_from(message.channel, limit=100):
             if log.author == message.author:
                 counter += 1