fix competition join

app/views/competitions/enroll.html.erb

@@ -1,7 +1,7 @@
 <div class="enroll-b">
   <div class="enroll-t" style="background:url(<%= @competition.identifier == 'gcc-dev-2018' ? '/images/educoder/competition/dev.jpg' : '/images/educoder/competition/anon.jpg' %>) no-repeat top center;">
     <div class="educontent">
-      <% if @competition.max_num > 1 %>
+      <% if @competition.competition_staffs.sum(:minimum) > 1 %>
         <p class="clearfix edu-txt-right mb30">
           <% unless User.current.logged? %>
             <%= link_to "创建战队", signin_path, :remote => true, :class => "enroll-in-b enroll-in-b-green fr" %>