Hi Kangarooo, thanks for taking the time to make this bug report and help make Ubuntu better.
This is a long standing bug in apt because of its use of carriage returns. Sometimes the line gets a bit shorter or longer based on the number of bytes or tasks left to do, so the s that was printed gets left over.
I'm fairly certain this has been around a long, long time, I can remember seeing it years ago.
Marking Confirmed, as I've seen this as well.
Setting Importance to Low, as this is purely cosmetic.