Fixed to_dict() bug for user-defined embed classes

discord/embeds.py

@@ -743,7 +743,7 @@ class Embed:
         # fmt: off
         result = {
             key[1:]: getattr(self, key)
-            for key in self.__slots__
+            for key in Embed.__slots__
             if key[0] == '_' and hasattr(self, key)
         }
         # fmt: on