doc: fix another broken link

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8763045

doc/articles/race_detector.html

@@ -142,7 +142,7 @@ $ GORACE="log_path=/tmp/race/report strip_path_prefix=/my/go/sources/" go test -
 
 <p>
 When you build with <code>-race</code> flag, the <code>go</code> command defines additional
-<a href="/pkg/go/build/#Build_Constraints">build tag</a> <code>race</code>.
+<a href="/pkg/go/build/#hdr-Build_Constraints">build tag</a> <code>race</code>.
 You can use the tag to exclude some code and tests when running the race detector.
 Some examples:
 </p>