Fix express router var name in example.

[IonoclastBrigham]

Part way down The router var magically changes from the declared app to an undeclared server, and then back again. Copy-paste error or partially-complete refactoring, probably? This won't compile in a lot of environments, and will definitely crash if you made it as far as running it.

I realize it's not actually meant to be run, but the example code should at least be correct, if not in fact useful.