change link to Conal Elliott's FRP explanation
change link to Conal Elliott's FRP explanation

fixes #136

phajduk/reactivex.github.io,phajduk/reactivex.github.io,ReactiveX/reactivex.github.io,BattleShipPark/reactivex.github.io,BattleShipPark/reactivex.github.io,ReactiveX/reactivex.github.io,phajduk/reactivex.github.io,BattleShipPark/reactivex.github.io,ReactiveX/reactivex.github.io