reflect: fix mistake in error message
reflect: fix mistake in error message

Pointed out by James Chalfant after submit of CL 6500065.

TBR=golang-dev
CC=golang-dev
http://codereview.appspot.com/6543067

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