fix English grammer

Author: zyxkad

src/syscall/js/js_test.go

@@ -176,7 +176,7 @@ func TestSymbol(t *testing.T) {
 }
 
 if js.Global().Call("Symbol").Equal(js.Global().Call("Symbol")) {
-t.Errorf("different Symbol instance is equal")
+t.Errorf("different Symbol instances are equal")
 }
 
 if iterSym := js.Global().Get("Symbol").Get("iterator"); iterSym.IsUndefined() {