fix: wrong logic

Author: wilr

src/EditableKeyValueField.php

@@ -6,7 +6,7 @@
 use SilverStripe\Forms\TextareaField;
 use SilverStripe\UserForms\Model\EditableFormField;
 
-if (class_exists(EditableFormField::class)) {
+if (!class_exists(EditableFormField::class)) {
     return;
 }