gc: fix stray %#N in error message
gc: fix stray %#N in error message

Fixes issue 2639.

R=rsc
CC=bradfitz, golang-dev
http://codereview.appspot.com/5489140

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