docs: minor: Add missing word 'it'

Signed-off-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
man: netsniff-ng: edits to the netsniff-ng man page

Various edits all over.

Signed-off-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>