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

lib/libnetgraph/netgraph.3

@@ -35,7 +35,7 @@
 .\" $FreeBSD$
 .\" $Whistle: netgraph.3,v 1.7 1999/01/25 07:14:06 archie Exp $
 .\"
-.Dd November 25, 2013
+.Dd May 15, 2020
 .Dt NETGRAPH 3
 .Os
 .Sh NAME
@@ -57,7 +57,7 @@
 .Sh LIBRARY
 .Lb libnetgraph
 .Sh SYNOPSIS
-.In netgraph/netgraph.h
+.In netgraph.h
 .Ft int
 .Fn NgMkSockNode "const char *name" "int *csp" "int *dsp"
 .Ft int