net/url: fix Encode doc comment
net/url: fix Encode doc comment

Encoded query strings are always sorted by key; the example wasn't.

R=golang-dev, dsymonds, minux.ma, bradfitz
CC=golang-dev
https://codereview.appspot.com/16430043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang