Update spiking_neural_systems.md

[jbrea]

It looks odd, when the membrane potential in an integrate-and-fire neuron overshoots the threshold.

Btw: I find it convenient to use ComponentArrays for parameters, such that I can write e.g. integrator.p.Vth instead of integrator.p[4], but I didn't apply this change here, as it would introduce a new dependency.

[ChrisRackauckas]

I think it would be fine to introduce the dependency. I would say it's good style and nice to show in an example.