Fix markup (.I is an unknown macro in this context).

usr.bin/awk/awk.1

@@ -1,4 +1,4 @@
-.\"	$NetBSD: awk.1,v 1.17 2008/05/26 02:07:25 dholland Exp $
+.\"	$NetBSD: awk.1,v 1.18 2009/03/09 14:10:12 joerg Exp $
 .\"
 .\" Copyright (C) Lucent Technologies 1997
 .\" All Rights Reserved
@@ -57,7 +57,7 @@ specified as
 With each pattern
 there can be an associated action that will be performed
 when a line of a
-.I file
+.Ar file
 matches the pattern.
 Each line is matched against the
 pattern portion of every pattern-action statement;