Formula update for Izhikevich_2007 Example

[DHRUVJ2003]

This pr is regarding issue #1476 .I have updated the formula for Izhikevich_2007 example neuron model. Let me know if any other changes are required to be made. (This is my first contribution to brian and open source , apologies for any mistake made from my end). Thanks

[mstimberg]

Thanks for the fix! Unfortunately, the fix makes the model not work anymore with the previously selected integration method (exact). I changed the method to euler so that it runs successfully. This example is a bit special, since it doesn't really matter what the neuron model does (this is why the error was in there in the first place…) – it is only used as a dummy neuron that is made to spike for each input. The interesting part of the model is the synaptic plasticity. On the good side, this means that the plot does not change with the change, so the complex stuff that I mentioned on the forum is not necessary here.