[Modify user] Fix. modify page should now have field set.

#1

src/modify.html

@@ -36,7 +36,7 @@
     function formHandler(listener) {
         listener.preventDefault();
         const formData = new FormData(formEl);
-        const field = formData.get("field");
+        const field = document.getElementById('field').value;
         if (field !== "") {
             const user = formData.get("user");
             const pass = formData.get("pass");