competition: fix join team fail bug

app/services/competitions/join_team_service.rb

@@ -24,7 +24,7 @@ class Competitions::JoinTeamService < ApplicationService
     raise Error, '您已加入该战队' if team.team_members.exists?(user_id: user.id)
 
     enrolled = competition.team_members.exists?(user_id: user.id)
-    if enrolled && (is_teacher && competition.teacher_multiple_limited?) || (!is_teacher && competition.member_multiple_limited?)
+    if enrolled && ((is_teacher && competition.teacher_multiple_limited?) || (!is_teacher && competition.member_multiple_limited?))
       raise Error, '您已加入其它战队'
     end