Fix duplicate offense messages in `RuboCop::Cop::Isucon::Mysql2::NPlusOneQuery`

[Go Sueyoshi]

rubocop-isucon/spec/rubocop/cop/isucon/mysql2/n_plus_one_query_spec.rb

Lines 137 to 139 in c4e9412

             teacher = db.xquery('SELECT * FROM `users` WHERE `id` = ?', course[:teacher_id]).first 
                       ^^ This looks like N+1 query. 
                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This looks like N+1 query.