Fix export_chat_invite_link not working for bots on channels/supergroups

Telegram still hasn't enabled this for bots...
Closes

pyrogram/client/methods/chats/export_chat_invite_link.py

@@ -52,11 +52,17 @@ class ExportChatInviteLink(BaseClient):
         """
         peer = self.resolve_peer(chat_id)
 
-        if isinstance(peer, (types.InputPeerChannel, types.InputPeerChat)):
+        if isinstance(peer, types.InputPeerChat):
             return self.send(
                 functions.messages.ExportChatInvite(
                     peer=peer
                 )
             ).link
+        elif isinstance(peer, types.InputPeerChannel):
+            return self.send(
+                functions.channels.ExportInvite(
+                    channel=peer
+                )
+            ).link
         else:
             raise ValueError('The chat_id "{}" belongs to a user'.format(chat_id))