Button to stop a stopwatch not working correctly!
There are several issues here. First and foremost, your button is wired
incorrectly. If you connect one end of the button to +5V, then the other
end should have a pushdown to GND, like here:

(Image from the Arduino Button example).
The second issue is the lack of debouncing. You should ignore the rising
edges you sense for a few milliseconds after you record one.
The third issue is the timekeeping. As explained by Sim Son, your
seconds will not be very accurate. It's better to just count the
milliseconds elapsed since the button press, and then break the count
into full seconds and remaining milliseconds.