Use Fn for function name.

lib/libc/gen/popen.3

@@ -1,4 +1,4 @@
-.\"	$NetBSD: popen.3,v 1.20 2015/01/21 08:35:31 wiz Exp $
+.\"	$NetBSD: popen.3,v 1.21 2017/02/21 19:53:20 abhinav Exp $
 .\"
 .\" Copyright (c) 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -57,15 +57,15 @@ a process by creating an IPC connection,
 forking,
 and invoking the shell.
 Historically,
-.Nm popen
+.Fn popen
 was implemented with a unidirectional pipe;
 hence many implementations of
-.Nm popen
+.Fn popen
 only allow the
 .Fa type
 argument to specify reading or writing, not both.
 Since
-.Nm popen
+.Fn popen
 is now implemented using sockets, the
 .Fa type
 may request a bidirectional data flow.