Update Quickstart observer to use 'events' instead of 'event'

The latest release of flecs uses an array for events in the description of observer

docs/Quickstart.md

@@ -1508,7 +1508,7 @@ An example of an observer with two components:
 ```c
 ecs_observer(world, {
  .filter.terms = { { ecs_id(Position) }, { ecs_id(Velocity) }},
- .event = EcsOnSet,
+ .events = { EcsOnSet },
  .callback = OnSetPosition
 });