doc: shorten project dashboard link

Fixes #3420.
(which was already fixed)

R=golang-dev, bradfitz, dsymonds
CC=golang-dev
https://golang.org/cl/5987043

doc/code.html

@@ -475,7 +475,7 @@ import "code.google.com/p/go.example/newmath"
 <p>
 This convention is the easiest way to make your Go packages available for
 others to use.
-The <a href="http://godashboard.appspot.com/project">Go Project Dashboard</a>
+The <a href="http://godashboard.appspot.com">Go Project Dashboard</a>
 is a list of external Go projects including programs and libraries.
 </p>