doc: fix date in weekly snapshot history

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4978057

doc/devel/weekly.html

@@ -14,7 +14,7 @@ hg pull
 hg update weekly.<i>YYYY-MM-DD</i>
 </pre>
 
-<h2 id="2011-09-06">2011-09-06</h2>
+<h2 id="2011-09-07">2011-09-07</h2>
 
 <pre>
 This weekly snapshot consists of improvements and bug fixes, including fixes