github: update contrib guide link in PR template

Even though we have moved the contribution process guide to our read the
docs, the template was still pointing to our redmine link.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 Make sure these boxes are signed before submitting your Pull Request -- thank you.
 
-- [ ] I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
+- [ ] I have read the contributing guide lines at https://suricata.readthedocs.io/en/latest/devguide/codebase/contributing/contribution-process.html
 - [ ] I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/
 - [ ] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)