Replace outdated links with correct links

The changes to rails@860ab28 have merged the bug report templates into one.
Since the template for the main branch has been removed, the link that was previously in CONTRIBUTING.md is invalid.

So I replaced them with the changed links.

CONTRIBUTING.md

@@ -15,9 +15,9 @@
 * If you're unable to find an open issue addressing the problem, [open a new one](https://github.com/rails/rails/issues/new). Be sure to include a **title and clear description**, as much relevant information as possible, and a **code sample** or an **executable test case** demonstrating the expected behavior that is not occurring.
 
 * If possible, use the relevant bug report templates to create the issue. Simply copy the content of the appropriate template into a .rb file, make the necessary changes to demonstrate the issue, and **paste the content into the issue description**:
-  * [**Active Record** (models, database) issues](https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_record_main.rb)
-  * [**Action Pack** (controllers, routing) issues](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_controller_main.rb)
-  * [**Generic template** for other issues](https://github.com/rails/rails/blob/main/guides/bug_report_templates/generic_main.rb)
+  * [**Active Record** (models, database) issues](https://github.com/rails/rails/blob/main/guides/bug_report_templates/active_record.rb)
+  * [**Action Pack** (controllers, routing) issues](https://github.com/rails/rails/blob/main/guides/bug_report_templates/action_controller.rb)
+  * [**Generic template** for other issues](https://github.com/rails/rails/blob/main/guides/bug_report_templates/generic.rb)
 
 * For more detailed information on submitting a bug report and creating an issue, visit our [reporting guidelines](https://edgeguides.rubyonrails.org/contributing_to_ruby_on_rails.html#reporting-an-issue).