Add an exercise about distance vector routing in the open questions of network.rst

[oliviertilmans]

The only question about distance vector protocols depicts a case where the protocol cannot converge (negative weight on a link) and where a routing loop is created. It would be useful to have a small exercise before it where the protocol converges normally.

It would allow to have one quick case to explain the basis of the protocol then after that to move on a more complicated case (i.e. routing loop)