Fix undefined method error

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/221052

changelogs/unreleased/221052-fix-custom-slashcommand-receiving-500.yml

+---
+title: Fix undefined method error
+merge_request: 34522
+author:
+type: fixed

lib/gitlab/slash_commands/presenters/help.rb

@@ -21,7 +21,7 @@ module Gitlab
               This chatops integration does not have any commands that can be
               executed.
 
-              #{footer}
+              #{help_footer}
             MESSAGE
           end