Header for FirewallD

Signed-off-by: Dan Schaper <dan.schaper@pi-hole.net>

docs/main/prerequesites.md

@@ -64,7 +64,7 @@ IP6Tables (IPv6)
 ```bash
 ip6tables -I INPUT -p udp -m udp --sport 546:547 --dport 546:547 -j ACCEPT
 ```
-FirewallD
+####FirewallD
 
 <TODO: Explain how to use FTLDNS with firewall-cmd>
 
@@ -72,4 +72,4 @@ FirewallD
 firewall-cmd --permanent --add-service=http --add-service=dns --add-service=dhcp --add-service=dhcpv6
 firewall-cmd --reload
 ```
-{!abbreviations.md!}
+{!abbreviations.md!}