Reference setprogname(3) in setproctitle(3)

The reference to setproctitle(3) in the setprogname(3) manual is already
in place.

MFC after:	3 days

lib/libc/gen/setproctitle.3

@@ -20,7 +20,7 @@
 .\" $FreeBSD$
 .\"
 .\" The following requests are required for all man pages.
-.Dd July 4, 2018
+.Dd November 13, 2020
 .Dt SETPROCTITLE 3
 .Os
 .Sh NAME
@@ -69,6 +69,7 @@ setproctitle("talking to %s", inet_ntoa(addr));
 .Sh SEE ALSO
 .Xr ps 1 ,
 .Xr w 1 ,
+.Xr setprogname 3 ,
 .Xr kvm 3 ,
 .Xr kvm_getargv 3 ,
 .Xr printf 3