doc: say that race detector is supported on freebsd

LGTM=ruiu
R=golang-codereviews, ruiu
CC=golang-codereviews
https://golang.org/cl/109100046

doc/articles/race_detector.html

@@ -377,7 +377,8 @@ func (w *Watchdog) Start() {
 <h2 id="Supported_Systems">Supported Systems</h2>
 
 <p>
-The race detector runs on <code>darwin/amd64</code>, <code>linux/amd64</code>, and <code>windows/amd64</code>.
+The race detector runs on <code>darwin/amd64</code>, <code>freebsd/amd64</code>,
+<code>linux/amd64</code>, and <code>windows/amd64</code>.
 </p>
 
 <h2 id="Runtime_Overheads">Runtime Overhead</h2>