fix: 字面量

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/onebot/event/request/OB11GroupRequest.ts

@@ -2,7 +2,7 @@ import { NapCatCore } from '@/core';
 import { OB11BaseRequestEvent } from './OB11BaseRequestEvent';
 
 export class OB11GroupRequestEvent extends OB11BaseRequestEvent {
-    override request_type = 'group';
+    override request_type: 'group' = 'group';
 
     group_id: number;
     user_id: number;