cmd/cue/cmd: correct module output

Fixes  #169

Change-Id: I13d25a1e1b3665f6c0a4fc17bdef637fe2f8d04f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3871
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cmd/cue/cmd/mod.go b/cmd/cue/cmd/mod.go
index 5e356f1..cc78991 100644
--- a/cmd/cue/cmd/mod.go
+++ b/cmd/cue/cmd/mod.go
@@ -84,7 +84,7 @@
 		if len(args) != 1 {
 			return fmt.Errorf("too many arguments")
 		}
-		module := args[0]
+		module = args[0]
 		u, err := url.Parse("https://" + module)
 		if err != nil {
 			return fmt.Errorf("invalid module name: %v", module)