doc: fix anchor tag for latest weekly.

R=golang-dev
TBR=adg
CC=golang-dev
https://golang.org/cl/5545068

doc/devel/weekly.html

@@ -14,7 +14,7 @@ hg pull
 hg update weekly.<i>YYYY-MM-DD</i>
 </pre>
 
-<h2 id="2012-01-14">2012-01-15</h2>
+<h2 id="2012-01-15">2012-01-15</h2>
 
 <pre>
 This weekly snapshot includes two package changes that may require changes to