doc: fix typo in go1.6 release date.

Change-Id: If15fdcd3cd49394a0c1dffd39fbbeede11081ccb
Reviewed-on: https://go-review.googlesource.com/19528Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/devel/release.html

@@ -30,7 +30,7 @@ to fix critical security problems in both Go 1.4 and Go 1.5 as they arise.
 See the <a href="/security">security policy</a> for more details.
 </p>
 
-<h2 id="go1.6">go1.6 (released 2015/02/17)</h2>
+<h2 id="go1.6">go1.6 (released 2016/02/17)</h2>
 
 <p>
 Go 1.6 is a major release of Go.