fix broken link [ci skip]

.github/ISSUE_TEMPLATE.md

@@ -9,7 +9,7 @@
 - [ ] I did install/upgraded to the latest version of pagy (or the latest `3.*` for pagy legacy)
 - [ ] I am providing at least one of the following working and self-sufficient code support that can reproduce this issue:
  - [ ] plain ruby file that can run as `ruby my-problem.rb`
- - [ ] edited copy of the single file [standalone_app.rb](https://github.com/ddnexus/pagy/blob/master/apps/standalone_app.rb)
+ - [ ] edited copy of the single file [standalone_app.rb](https://github.com/ddnexus/pagy/blob/master/apps/standalone_app.ru)
  - [ ] link of my own branch forked from one branch of the [pagy-rails app](https://github.com/benkoshy/pagy-rails)
  - [ ] link of a standalone `docker image` downloadable from a docker repository (e.g. dockerhub.com)
  - [ ] `docker-compose` file that can be run with `docker-compose up` (and doesn't need to be built because it uses only docker images and no local context)