Correct multicast address used in pfsync(4)
As stated in src/sys/netinet/in.h L395, address used is 224.0.0.240.

PR: 216885
Submitted by: Matthias Ordner <matthias.ordner AT noris DOT net>
Reviewed by: kp
Approved by: bcr (mentor)
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D12011