Convert two missing places to f-strings

discord/abc.py

@@ -359,7 +359,7 @@ class GuildChannel:
     @property
     def mention(self):
         """:class:`str`: The string that allows you to mention the channel."""
-        return '<#%s>' % self.id
+        return f'<#{self.id}>'
 
     @property
     def created_at(self):

discord/invite.py

@@ -78,7 +78,7 @@ class PartialInviteChannel:
     @property
     def mention(self):
         """:class:`str`: The string that allows you to mention the channel."""
-        return '<#%s>' % self.id
+        return f'<#{self.id}>'
 
     @property
     def created_at(self):