godoc: fix codewalk handler

For URLs ending with /, the handler did not work, trying to append
".xml" to the path.

For instance, the "Share Memory by Communicating" returned the
following error:

        open /Users/francisco.souza/lib/go/doc/codewalk/sharemem/.xml: no such file or directory

R=adg, minux.ma
CC=golang-dev
https://golang.org/cl/5797065

src/cmd/godoc/codewalk.go

@@ -53,7 +53,9 @@ func codewalk(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Otherwise append .xml and hope to find
-	// a codewalk description.
+	// a codewalk description, but before trim
+	// the trailing /.
+	abspath = strings.TrimRight(abspath, "/")
 	cw, err := loadCodewalk(abspath + ".xml")
 	if err != nil {
 		log.Print(err)