doc/devel/release.html: document go1.6.3 doesn't actually support macOS Sierra

Updates #17824.

Change-Id: I73cf89c21b418158c7014c3271cd1103a17a5c86
Reviewed-on: https://go-review.googlesource.com/32882Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/devel/release.html

@@ -83,7 +83,7 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.2">Go
 <p>
 go1.6.3 (released 2016/07/17) includes security fixes to the
 <code>net/http/cgi</code> package and <code>net/http</code> package when used in
-a CGI environment. This release also adds support for macOS Sierra.
+a CGI environment.
 See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.3">Go
 1.6.3 milestone</a> on our issue tracker for details.
 </p>