core: better pid sort method

packages/hydrooj/src/handler/problem.ts

@@ -148,7 +148,7 @@ export class ProblemMainHandler extends ProblemHandler {
         await bus.serial('problem/list', query, this);
         // eslint-disable-next-line prefer-const
         let [pdocs, ppcount, pcount] = await paginate(
-            problem.getMulti(domainId, query).sort({ pid: 1, docId: 1 }),
+            problem.getMulti(domainId, query).sort({ sort: 1, docId: 1 }),
             page,
             system.get('pagination.problem'),
         );