man dummynet: point to dnctl instead of ipfw

Dummynet configuration is ideally done through dnctl now. While ipfw
still works dnctl is preferred now that dummynet can also be used with
pf.

MFC after:	2 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D31902

share/man/man4/dummynet.4

 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 28, 2002
+.Dd September 10, 2021
 .Dt DUMMYNET 4
 .Os
 .Sh NAME
@@ -18,9 +18,9 @@ management policies, and emulating delays and losses.
 The user interface for
 .Nm
 is implemented by the
-.Xr ipfw 8
+.Xr dnctl 8
 utility, so please refer to the
-.Xr ipfw 8
+.Xr dnctl 8
 manpage for a complete description of the
 .Nm
 capabilities and how to use it.
@@ -61,6 +61,7 @@ pipes.
 .Xr if_bridge 4 ,
 .Xr ip 4 ,
 .Xr ipfw 8 ,
+.Xr dnctl 8 ,
 .Xr sysctl 8
 .Sh HISTORY
 The