go-tour: remove "not even optional" about parens on if statements
go-tour: remove "not even optional" about parens on if statements

(They are optional.)

Fixes issue 52.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/10861043

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