doc: fix installation $GOOS choices

The $GOOS choices text and combinations list should match.

R=adg
CC=golang-dev
https://golang.org/cl/3485046

doc/install.html

@@ -340,7 +340,8 @@ to override the defaults.
 	Choices for <code>$GOOS</code> are <code>linux</code>,
 	<code>freebsd</code>,
 	<code>darwin</code> (Mac OS X 10.5 or 10.6),
-	and <code>nacl</code> (Native Client, an incomplete port).
+	<code>nacl</code> (Native Client, an incomplete port),
+	and <code>windows</code> (Windows, an incomplete port).
 	Choices for <code>$GOARCH</code> are <code>amd64</code> (64-bit x86, the most mature port),
 	<code>386</code> (32-bit x86), and
 	<code>arm</code> (32-bit ARM, an incomplete port).