Fix a typo.

models/problem.js

@@ -500,7 +500,7 @@ class Problem extends Model {
   async resetSubmissionCount() {
     await syzoj.utils.lock(['Problem::resetSubmissionCount', this.id], async () => {
       this.submit_num = await JudgeState.count({ score: { $not: null }, problem_id: this.id, type: { $not: 1 } });
-      this.ac_num = await JudgeState.count({ score: 100, problem_id: this.id, $type: { $not: 1 } });
+      this.ac_num = await JudgeState.count({ score: 100, problem_id: this.id, type: { $not: 1 } });
       await this.save();
     });
   }