selinux: Update SELinux SCTP documentation

Update SELinux-sctp.rst "SCTP Peer Labeling" section to reflect
how the association permission is validated.

Reported-by: Dominick Grift <dac.override@gmail.com>
Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>