changed payment code to demo code

backend/core/templates/admin/generate_codes.html

@@ -2,7 +2,7 @@
 
 {% block content %}
   <div id="content-main">
-    <h2>Generate Payment Codes</h2>
+    <h2>Generate Demo Codes</h2>
     <form method="post">{% csrf_token %}
       {{ form.as_p }}
       <input type="submit" name="apply" value="Generate" class="default">