testing: Fix typo on Logf doc comment.
testing: Fix typo on Logf doc comment.

R=rsc, r2
CC=golang-dev
http://codereview.appspot.com/4167064

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