[x/tour] gotour: resolve ambiguous reference to PI
[x/tour] 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
https://golang.org/cl/5311084
X-Tour-Commit: 8156e93d804f063bc7cb01f21d441c2b6e21ce29

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website