Fix example in iptables module (#54906)

In the documentation the rule_num parameter works only in conjunction with `action` set to `insert` - fix the example with the rule_num parameter.

+label: docsite_pr

lib/ansible/modules/system/iptables.py

@@ -399,6 +399,7 @@ EXAMPLES = r'''
     protocol: tcp
     destination_port: 8080
     jump: ACCEPT
+    action: insert
     rule_num: 5
 
 - name: Set the policy for the INPUT chain to DROP