Fix inaccurate instructions around import

cms/templates/import.html

@@ -21,7 +21,7 @@
       <div class="description">
         <p><strong>Importing a new course will delete all content currently associated with your course
         and replace it with the contents of the uploaded file.</strong></p>
-        <p>File uploads must be gzipped tar files (.tar.gz or .tgz) containing, at a minimum, a <code>course.xml</code> file.</p>
+        <p>File uploads must be gzipped tar files (.tar.gz) containing, at a minimum, a <code>course.xml</code> file.</p>
         <p>Please note that if your course has any problems with auto-generated <code>url_name</code> nodes,
         re-importing your course could cause the loss of student data associated with those problems.</p>
       </div>