Update schematype.js

Fixed documentation for returning a promise from a validator to match documentation at http://mongoosejs.com/docs/validation.html.

lib/schematype.js

@@ -419,9 +419,8 @@ SchemaType.prototype.get = function(fn) {
  *
  *     // Can also return a promise
  *     schema.path('name').validate({
- *       isAsync: true,
- *       validator: function (value, respond) {
- *         return new Promise(resolve => {
+ *       validator: function (value) {
+ *         return new Promise(function (resolve, reject) {
  *           resolve(false); // validation failed
  *         });
  *       }