competition: fix enroll api error

app/services/competitions/create_personal_team_service.rb

@@ -21,7 +21,7 @@ class Competitions::CreatePersonalTeamService < ApplicationService
 
     ActiveRecord::Base.transaction do
       team = competition.competition_teams.create!(name: user.real_name, user_id: user.id)
-      team.team_members.create!(competition_id: competition, user_id: user.id, role: 1, is_teacher: is_teacher)
+      team.team_members.create!(competition_id: competition.id, user_id: user.id, role: 1, is_teacher: is_teacher)
     end
   end
 end