remove warning about slug not being preserved in edit topic view

app/templates/mod/edit_topic.html

@@ -2,7 +2,7 @@
 {%- extends 'base.html' -%}
 {%- block title -%}editing topic {{topic.name}}{%- endblock -%}
 {%- block content -%}
-{{subheader('Editing topic %s' % topic.name, 'To preserve history, the URL of the topic can not be changed.')}}
+{{subheader('Editing topic %s' % topic.name)}}
 <form class="plank primary-bg full-width" method="POST">
   <label for="name">Name</label>
   <input type="text" id="name" name="name" required value="{{topic.name}}">