godoc: fix playground url

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5685070

doc/root.html

@@ -118,7 +118,7 @@ function init() {
 		"outputEl":      "#output",
 		"runEl":         "#run",
 		"shareEl":       "#share",
-		"shareRedirect": "http://play.golang.org/"
+		"shareRedirect": "http://play.golang.org/p/"
 	});
 }