Fixes TypeError in bot.py (#240)

Currently using this will TypeError as the parameter (`self._modules`) is initialised as a list, not a dict as needed.

Author: AbstractUmbra

twitchio/ext/commands/bot.py

@@ -127,7 +127,7 @@ def from_client_credentials(
         )  # The only reason we're even creating this is to avoid attribute errors
         self._events = {}
         self._waiting = []
-        self._modules = []
+        self._modules = {}
         self._prefix = prefix
         self._cogs = {}
         self._commands = {}