tour: fix formulae for sqrt/cbrt exercises
tour: fix formulae for sqrt/cbrt exercises

R=rsc, thakis
CC=golang-dev
http://codereview.appspot.com/5248053

BenjaminTsai/go-tour-zh-tw,BenjaminTsai/go-tour-zh-tw,golang-tr/go-tour-tr,golang-tr/go-tour-tr