gotest: update URL printed by failure message.

Fixes #677.

R=rsc
CC=golang-dev
https://golang.org/cl/834046

src/cmd/gotest/gotest

@@ -19,7 +19,7 @@ GOBIN="${GOBIN:-$HOME/bin}"
 _GC=$GC	# Make.$GOARCH will overwrite this
 
 if [ ! -f [Mm]akefile ]; then
-	echo 'please create a Makefile for gotest; see http://golang.org/doc/contribute.html (sic) for details' 1>&2
+	echo 'please create a Makefile for gotest; see http://golang.org/doc/code.html for details' 1>&2
 	exit 2
 fi