blackhole.4: improve man page

Not the connection is dropped, but the incoming SYN segment.

Reviewed by: concussious.bugzilla_runbox.com
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45296

(cherry picked from commit a45a9e61b498393b05f3e1e2e3e6131c7b484dc4)