Fix create milestone with deadline bug (#462)

Fix #461

Reviewed-on: https://gitea.com/gitea/tea/pulls/462
Reviewed-by: Norwin <noerw@noreply.gitea.io>
Reviewed-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-committed-by: Lunny Xiao <xiaolunwen@gmail.com>

cmd/milestones/create.go

@@ -55,7 +55,7 @@ func runMilestonesCreate(cmd *cli.Context) error {
 	deadline := &time.Time{}
 	if date != "" {
 		t, err := dateparse.ParseAny(date)
-		if err == nil {
+		if err != nil {
 			return err
 		}
 		deadline = &t