Fix invalid format specifier in PartialWebhookState.__getattr__

The message for the AttributeError raised by the __getattr__ method of
the PartialWebhookState class is formatted using `str.format`. However,
the placeholder contained a stray ":" before the !r specifier. This
caused a ValueError("Invalid format specifier") to be raised whenever
this method was called in lieu of the AttributeError that is intended
to be raised.

discord/webhook.py

@@ -357,7 +357,7 @@ class _PartialWebhookState:
         return _FriendlyHttpAttributeErrorHelper()
 
     def __getattr__(self, attr):
-        raise AttributeError('PartialWebhookState does not support {0:!r}.'.format(attr))
+        raise AttributeError('PartialWebhookState does not support {0!r}.'.format(attr))
 
 class Webhook:
     """Represents a Discord webhook.