Fix At argument and remove superfluous Pp.

sbin/shutdown/shutdown.8

@@ -1,4 +1,4 @@
-.\"	$NetBSD: shutdown.8,v 1.34 2022/07/01 16:45:12 kre Exp $
+.\"	$NetBSD: shutdown.8,v 1.35 2022/07/01 16:58:02 wiz Exp $
 .\"
 .\" Copyright (c) 1988, 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -176,7 +176,6 @@ is supplied as the only argument after the time, the warning message is read
 from the standard input.
 .El
 .Sh BEHAVIOR
-.Pp
 At intervals, becoming more frequent as apocalypse approaches
 and starting at ten hours before shutdown, warning messages are displayed
 on the terminals of all users logged in.
@@ -245,6 +244,6 @@ a colon (``:'') for backward compatibility.
 A
 .Nm
 command was originally written by Ian Johnstone for UNSW's modified
-.At "6th Edn" ,
+.At v6 ,
 modified, and then incorporated in
 .Bx 4.1 .