Fixed to_dict() bug for user-defined embed classes

Author: cyberoby

discord/embeds.py

@@ -743,7 +743,7 @@ def to_dict(self) -> EmbedData:
         # 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