use RST, not markdown, in the changelog fragment

Author: sdwilsh

changelogs/fragments/diff-mode-firewall_rules.yml

@@ -1,3 +1,3 @@
 ---
 major_changes:
-  - Support [diff mode](https://docs.ansible.com/ansible/latest/user_guide/playbooks_checkmode.html#using-diff-mode) in `vyos_firewall_rules`
+  - Support `diff mode <https://docs.ansible.com/ansible/latest/user_guide/playbooks_checkmode.html#using-diff-mode>` in ``vyos_firewall_rules``