testing: Fix typo on Logf doc comment.

R=rsc, r2
CC=golang-dev
https://golang.org/cl/4167064

src/pkg/testing/testing.go

@@ -92,7 +92,7 @@ func (t *T) FailNow() {
 // and records the text in the error log.
 func (t *T) Log(args ...interface{}) { t.errors += "\t" + tabify(fmt.Sprintln(args...)) }
 
-// Log formats its arguments according to the format, analogous to Printf(),
+// Logf formats its arguments according to the format, analogous to Printf(),
 // and records the text in the error log.
 func (t *T) Logf(format string, args ...interface{}) {
 	t.errors += "\t" + tabify(fmt.Sprintf(format, args...))