doc: fix reference to non-existing method #293
                
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It's confusing that up-to-date documentation (https://www.pymunk.org/en/latest/pymunk.html#pymunk.Poly.collision_type) references add_collision_handler, which seems to be removed (also this isn't found in changelogs! Maybe happened in 7.0.0?)
on_collisionseems to document collision_types, so reference that instead.