Fixed wrong units for specific heat in API doc for Props and State classes

API doc for `Props` and `State` classes contains wrong units for specific heat. This should be [kJ/kg/K] instead of [kJ/kg].

* http://coolprop.sourceforge.net/apidoc/CoolProp.html#CoolProp.CoolProp.Props
* http://coolprop.sourceforge.net/apidoc/State.html#CoolProp.State.State.get_cp