spec: better wording re: receiving from closed channels
spec: better wording re: receiving from closed channels

As suggested by ej@evanjones.ca.

Fixes issue 5604.

R=r
CC=golang-dev
https://codereview.appspot.com/9899043

apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme