Fix grammar error pointed out by Peter Seebach in PR#1846

lib/libc/sys/madvise.2

@@ -1,4 +1,4 @@
-.\"	$NetBSD: madvise.2,v 1.6 1995/10/12 15:41:03 jtc Exp $
+.\"	$NetBSD: madvise.2,v 1.7 1995/12/27 21:17:02 jtc Exp $
 .\"
 .\" Copyright (c) 1991, 1993
 .\"	The Regents of the University of California.  All rights reserved.
@@ -38,7 +38,7 @@
 .Os
 .Sh NAME
 .Nm madvise
-.Nd give advise about use of memory
+.Nd give advice about use of memory
 .Sh SYNOPSIS
 .Fd #include <sys/types.h>
 .Fd #include <sys/mman.h>