runtime/race: fix the link for the race detector.

LGTM=bradfitz
R=golang-dev, bradfitz
CC=golang-codereviews
https://golang.org/cl/100330043

src/pkg/runtime/race/doc.go

@@ -5,5 +5,5 @@
 // Package race implements data race detection logic.
 // No public interface is provided.
 // For details about the race detector see
-// http://golang.org/doc/articles/race_detector.html
+// http://blog.golang.org/race-detector
 package race