修改错别字

readme.md

@@ -143,8 +143,8 @@ class MyModel extends base\MyModel
     public function fields()
     {
         $fields = [];
-        foreach (self::BIT_FLAGS as $bigFlag) {
-            $fields = ArrayHelper::merge($fields, array_combine($bigFlag, $bigFlag));
+        foreach (self::BIT_FLAGS as $bitFlag) {
+            $fields = ArrayHelper::merge($fields, array_combine($bitFlag, $bitFlag));
         }
         return ArrayHelper::merge(parent::fields(), $fields);
     }