validate callback 返回类型错误 (#2650)

如果返回的是 Boolean 类型，做 if 判断的时候，结果会被认为永远是 true

Author: ken1987

types/form-model/form.d.ts

@@ -153,7 +153,7 @@ export declare class FormModel extends AntdComponent {
    * and an object containing all fields that fail the validation. Returns a promise if callback is omitted
    * @type Function
    */
-  validate: (callback?: (boolean: Boolean, object: Object) => void) => void | Promise<any>;
+  validate: (callback?: (boolean: boolean, object: Object) => void) => void | Promise<any>;
 
   /**
    * validate one or several form items