examples: fix timers example
examples: fix timers example

It was calling delete_timer after shutting down the timer module.
I'm not sure it was supported back at at time, but it now asserts
(and rightly so).

DragonMinded/libdragon,sp1187/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,sp1187/libdragon,sp1187/libdragon