In r343587 a simple port filter as sysctl tunable was added to siftr.
The new sysctl was not added to the siftr.4 man page at the time.
This updates the man page, and removes one left over trailing whitespace.

Submitted by: Richard Scheffenegger
Reviewed by: bcr@
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D21619