Docs fix

Fixed error in Migration FAQ.

docs/source/migration_1_to_2.rst

@@ -68,6 +68,9 @@ Also you can bind your own filters for using as keyword arguments:
     class MyFilter(BoundFilter):
         key = 'is_admin'
         
+        def __init__(self, is_admin):
+            pass
+            
         async def check(self, message: types.Message):
             member = await bot.get_chat_member(message.chat.id, message.from_user.id)
             return member.is_admin()