Add forum post rename system message

discord/message.py

@@ -1735,6 +1735,9 @@ class Message(PartialMessage, Hashable):
                 return f'{self.author.name} removed {self.mentions[0].name} from the thread.'
 
         if self.type is MessageType.channel_name_change:
+            if getattr(self.channel, 'parent', self.channel).type is ChannelType.forum:
+                return f'{self.author.name} changed the post title: **{self.content}**'
+            else:
                 return f'{self.author.name} changed the channel name: **{self.content}**'
 
         if self.type is MessageType.channel_icon_change: