cmd/toolstash: fix package name in godoc hint

This package has been moved since then.

Change-Id: Ibf0044cf4fcef47ae15301c2880ff129a8829516
Reviewed-on: https://go-review.googlesource.com/41856
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/toolstash/main.go b/cmd/toolstash/main.go
index a77171f..e5d226d 100644
--- a/cmd/toolstash/main.go
+++ b/cmd/toolstash/main.go
@@ -147,7 +147,7 @@
 	toolstash compile x.go
 	toolstash -cmp compile x.go
 
-For details, godoc rsc.io/toolstash
+For details, godoc golang.org/x/tools/cmd/toolstash
 `
 
 func usage() {