docs(document): fix code sample that errors with "Cannot set properties of undefined"

Fix #15584

Author: vkarpov15

lib/document.js

@@ -4070,15 +4070,17 @@ Document.prototype.$__toObjectShallow = function $__toObjectShallow(schemaFields
  *
  * If you want to skip transformations, use `transform: false`:
  *
- *     schema.options.toObject.hide = '_id';
- *     schema.options.toObject.transform = function (doc, ret, options) {
- *       if (options.hide) {
- *         options.hide.split(' ').forEach(function (prop) {
- *           delete ret[prop];
- *         });
+ *     schema.options.toObject = {
+ *       hide: '_id',
+ *       transform: function(doc, ret, options) {
+ *         if (options.hide) {
+ *           options.hide.split(' ').forEach(function(prop) {
+ *             delete ret[prop];
+ *           });
+ *         }
+ *         return ret;
  *       }
- *       return ret;
- *     }
+ *     };
  *
  *     const doc = new Doc({ _id: 'anId', secret: 47, name: 'Wreck-it Ralph' });
  *     doc.toObject();                                        // { secret: 47, name: 'Wreck-it Ralph' }