Broken link to `Valve.set_valve_coefficient` docs

[bryanwweber]

https://cantera.org/science/reactors.html#id3

[speth]

I think this was fixed by @chinahg's reorg of the Reactor documentation in #189. On the current page (https://cantera.org/science/reactors/reactors.html#reactor-peripherals), all the links related to Valve seem to go to the right place.