doc: fix typo in race_detector.html

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8625044

doc/articles/race_detector.html

@@ -34,7 +34,7 @@ func main() {
 <h2 id="Usage">Usage</h2>
 
 <p>
-To help diangose such bugs, Go includes a built-in data race detector.
+To help diagnose such bugs, Go includes a built-in data race detector.
 To use it, add the <code>-race</code> flag to the go command:
 </p>