ngctl.8: fix incorrect synopsis

Tiny manpage bug I caused in D50241
I left a line in the manpage that shouldn't be there.

MFC after: 3 days
Fixes: 72d01e62b082 ("netgraph: teach ngctl to attach and run itself in a jail")
Reviewed by: des, markj
Differential Revision: https://reviews.freebsd.org/D55485