Update core.js

Author: andi34

src/js/core.js

@@ -620,7 +620,7 @@ const photoBooth = (function () {
     api.printImage = function (imageSrc, cb) {
         const errormsg = i18n('error');
 
-        if !(printing) {
+        if (!printing) {
             modal.open('#print_mesg');
             printing = true;
             setTimeout(function () {