gluon-ebtables-filter-multicast: fix comment char in Lua code

It seems this mistake didn't cause an error because of Lua's support for
shebang lines.

package/gluon-ebtables-filter-multicast/files/lib/gluon/ebtables/110-mcast-allow-arp

-# Bridge loop avoidance
+-- Bridge loop avoidance
 rule 'MULTICAST_OUT -p ARP --arp-opcode Reply --arp-gratuitous --arp-mac-dst ff:43:05:00:00:00/ff:ff:ff:fc:00:00 -j RETURN'
 rule 'MULTICAST_OUT -p ARP --arp-opcode Reply --arp-gratuitous --arp-mac-dst ff:43:05:05:00:00/ff:ff:ff:ff:00:00 -j RETURN'