MFC rev 1.73 newfs.8

Replace reference from vinum.8 to gvinum.8, it was advised in the PR to
  replace this with vinum.4, but that's the kernel interface manual, which
  is not appropriate in my understanding.  I think that gvinum is a suitable
  replacement for this.

  PR:             docs/121938
  Submitted by:   "Federico" <federicogalvezdurand at yahoo dot com>
  MFC after:      3 days

sbin/newfs/newfs.8

@@ -28,7 +28,7 @@
 .\"     @(#)newfs.8	8.6 (Berkeley) 5/3/95
 .\" $FreeBSD$
 .\"
-.Dd January 21, 2005
+.Dd March 21, 2008
 .Dt NEWFS 8
 .Os
 .Sh NAME
@@ -244,7 +244,7 @@ on file systems that contain many small files.
 .Xr fsck 8 ,
 .Xr mount 8 ,
 .Xr tunefs 8 ,
-.Xr vinum 8
+.Xr gvinum 8
 .Rs
 .%A M. McKusick
 .%A W. Joy