cmd/go: fix TestModSyncPrintJson test

Change-Id: I043150aac08014f6e68c4b4d75d8f9cb59b458ca
Reviewed-on: https://go-review.googlesource.com/121880
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/vendor/cmd/go/vgo_test.go b/vendor/cmd/go/vgo_test.go
index f374fc0..ef74dce 100644
--- a/vendor/cmd/go/vgo_test.go
+++ b/vendor/cmd/go/vgo_test.go
@@ -841,7 +841,7 @@
 		package x
 		import "github.com/gorilla/mux"
 		func main() {
-			_ := mux.NewRouter()
+			_ = mux.NewRouter()
 		}`), 0666))
 	tg.must(ioutil.WriteFile(tg.path("x/go.mod"), []byte("module x"), 0666))
 	tg.cd(tg.path("x"))