Fix SYNPOSIS section to point to the proper include directive.

netgraph(3) points to #include <netgraph/netgraph.h>, which is kernel only.
The man page refers to the user-space part of the netgraph module, which is
located in <netgraph.h>.

Submitted by: lutz_donnerhacke.de
Approved by: bcr
Differential Revision: https://reviews.freebsd.org/D23814