Fix format string in documentation.

docs/quickstart.rst

@@ -23,7 +23,7 @@ It looks something like this:
 
     @client.event
     async def on_ready():
-        print('We have logged in as {0.user}'.format(self))
+        print('We have logged in as {0.user}'.format(client))
 
     @client.event
     async def on_message(message):