gotour: resolve ambiguous reference to PI
gotour: resolve ambiguous reference to PI

Some people were confused when math.Pi worked, but not math.PI.
Making this paragraph abstract helps.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5311084

Go-zh/tour-old,Go-zh/tour-old,minux/go-tour-zh,minux/go-tour-zh