core: fix tsdoc.endAt display

packages/hydrooj/package.json

@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "3.15.2",
+    "version": "3.15.3",
     "bin": "bin/hydrooj.js",
     "main": "src/loader",
     "module": "src/loader",

packages/hydrooj/src/handler/contest.ts

@@ -102,7 +102,7 @@ export class ContestDetailBaseHandler extends Handler {
                 throw new ForbiddenError('You are not assigned.');
             }
         }
-        if (this.tdoc.duration && this.tsdoc?.attend) {
+        if (this.tdoc.duration && this.tsdoc?.startAt) {
             this.tsdoc.endAt = moment(this.tsdoc.startAt).add(this.tdoc.duration, 'hours').toDate();
         }
     }