Better StopTime for Modelica.Electrical.Analog.Examples.DifferenceAmplifier

[HansOlsson]

It was changed from 1s to 1e-8s due to https://github.com/modelica/ModelicaStandardLibrary/pull/3625

However, I believe it would be better to simulate to 1e-7s (and also have that in the documentation) - since the input is an exponentially decaying sine-signals. If you just simulate to 1e-8s you'll miss the decay, and R4.n.v doesn't stabilize around the new mean.

5e-8s would also work, whereas 5e-7s would be too much. No need to change release.