core: fix broken link

packages/hydrooj/src/model/builtin.ts

@@ -262,7 +262,8 @@ PRIV.PRIV_DEFAULT = PRIV.PRIV_USER_PROFILE
     + PRIV.PRIV_REGISTER_USER
     + PRIV.PRIV_READ_PRETEST_DATA_SELF
     + PRIV.PRIV_CREATE_FILE
-    + PRIV.PRIV_DELETE_FILE_SELF;
+    + PRIV.PRIV_DELETE_FILE_SELF
+    + PRIV.PRIV_SEND_MESSAGE;
 
 // people whose rank is less than 1% will get Level 10
 export const LEVELS = [100, 90, 70, 55, 40, 30, 20, 10, 5, 2, 1];